Off Elizabeth Island, which, however, consists of raised coral, Captain Beechey (page 45, 4to edition) describes
three ledges: the first had an easy slope from the beach to a distance of about fifty yards: the second extended
two hundred yards with twenty−five fathoms on it, and then ended abruptly, like the first; and immediately
beyond this there was no bottom with two hundred fathoms.) Here then occurs a difficulty; can sand
accumulate on a slope, which, in some cases, appears to exceed fifty−five degrees? It must be observed, that I
speak of slopes where soundings were obtained, and not of such cases, as that of Cardoo, where the nature of
the bottom is unknown, and where its inclination must be nearly vertical. M. Elie de Beaumont ("Memoires
pour servir a une description Geolog. de France," tome iv., page 216.) has argued, and there is no higher
authority on this subject, from the inclination at which snow slides down in avalanches, that a bed of sand or
mud cannot be formed at a greater angle than thirty degrees. Considering the number of soundings on sand,
obtained round the Maldiva and Chagos atolls, which appears to indicate a greater angle, and the extreme
abruptness of the sand−banks in the West Indies, as will be mentioned in the Appendix, I must conclude that
the adhesive property of wet sand counteracts its gravity, in a much greater ratio than has been allowed for by
M. Elie de Beaumont. From the facility with which calcareous sand becomes agglutinated, it is not necessary
to suppose that the bed of loose sand is thick.
Captain Beechey has observed, that the submarine slope is much less at the extremities of the more elongated
atolls in the Low Archipelago, than at their sides; in speaking of Ducie's Island he says (Beechey's "Voyage,"
4to edition, page 44.) the buttress, as it may be called, which "has the most powerful enemy (the S.W. swell)
to oppose, is carried out much further, and with less abruptness than the other." In some cases, the less
inclination of a certain part of the external slope, for instance of the northern extremities of the two Keeling
atolls, is caused by a prevailing current which there accumulates a bed of sand. Where the water is perfectly
tranquil, as within a lagoon, the reefs generally grow up perpendicularly, and sometimes even overhang their
bases; on the other hand, on the leeward side of Mauritius, where the water is generally tranquil, although not
invariably so, the reef is very gently inclined. Hence it appears that the exterior angle varies much;
nevertheless in the close similarity in form between the sections of Keeling atoll and of the atolls in the Low
Archipelago, in the general steepness of the reefs of the Maldiva and Chagos atolls, and in the
perpendicularity of those rising out of water always tranquil, we may discern the effects of uniform laws; but
from the complex action of the surf and currents, on the growing powers of the coral and on the deposition of
sediment, we can by no means follow out all the results.
Where islets have been formed on the reef, that part which I have sometimes called the "flat" and which is
partly dry at low water, appears similar in every atoll. In the Marshall group in the North Pacific, it may be
inferred from Chamisso's description, that the reef, where islets have not been formed on it, slopes gently
from the external margin to the shores of the lagoon; Flinders states that the Australian barrier has a similar
inclination inwards, and I have no doubt it is of general occurrence, although, according to Ehrenberg, the
reefs of the Red Sea offer an exception. Chamisso observes that "the red colour of the reef (at the Marshall
atolls) under the breakers is caused by a Nullipora, which covers the stone WHEREVER THE WAVES
BEAT; and, under favourable circumstances, assumes a stalactical form,"−−a description perfectly applicable
to the margin of Keeling atoll. (Kotzebue's "First Voyage," volume iii., page 142. Near Porto Praya, in the
Cape de Verde Islands, some basaltic rocks, lashed by no inconsiderable surf, were completely enveloped
with a layer of Nulliporae. The entire surface over many square inches, was coloured of a peach−blossomed
red; the layer, however, was of no greater thickness than paper. Another kind, in the form of projecting knobs,
grew in the same situation. These Nulliporae are closely related to those described on the coral−reefs, but I
believe are of different species.) Although Chamisso does not state that the masses of Nulliporae form points
or a mound, higher than the flat, yet I believe that this is the case; for Kotzebue (Kotzebue, "First Voyage,"
volume ii., page 16. Lieutenant Nelson, in his excellent memoir in the Geological Transactions (volume ii.,
page 105), alludes to the rocky points mentioned by Kotzebue, and infers that they consist of Serpulae, which
compose incrusting masses on the reefs of Bermudas, as they likewise do on a sandstone bar off the coast of
Brazil (which I have described in "London Phil. Journal," October 1841). These masses of Serpulae hold the
same position, relatively to the action of the sea, with the Nulliporae on the coral−reefs in the Indian and
Pacific Oceans.), in another part, speaks of the rocks on the edge of the reef "as visible for about two feet at
low water," and these rocks we may feel quite certain are not formed of true coral (Captain Moresby, in his
valuable paper "on the Northern atolls of Maldivas" ("Geographical Journal", volume v.), says that the edges
of the reefs there stand above water at low spring−tides.) Whether a smooth convex mound of Nulliporae, like
that which appears as if artificially constructed to protect the margin of Keeling Island, is of frequent
occurrence round atolls, I know not; but we shall presently meet with it, under precisely the same form, on the
outer edge of the "barrier−reefs" which encircle the Society Islands.
There appears to be scarcely a feature in the structure of Keeling reef, which is not of common, if not of
universal occurrence, in other atolls. Thus Chamisso describes (Kotzebue's "First Voyage," volume iii., page
144.) a layer of coarse conglomerate, outside the islets round the Marshall atolls which "appears on its upper
surface uneven and eaten away." From drawings, with appended remarks, of Diego Garcia in the Chagos
group and of several of the Maldiva atolls, shown me by Captain Moresby (see also Moresby on the Northern
atolls of the Maldivas, "Geographical Journal", volume v., page 400.), it is evident that their outer coasts are
subject to the same round of decay and renovation as those of Keeling atoll. From the description of the atolls
in the Low Archipelago, given in Captain Beechey's "Voyage," it is not apparent that any conglomerate
coral−rock was there observed.
The lagoon in Keeling atoll is shallow; in the atolls of the Low Archipelago the depth varies from 20 to 38
fathoms, and in the Marshall Group, according to Chamisso, from 30 to 35; in the Caroline atolls it is only a
little less. Within the Maldiva atolls there are large spaces with 45 fathoms, and some soundings are laid down
of 49 fathoms. The greater part of the bottom in most lagoons, is formed of sediment; large spaces have
exactly the same depth, or the depth varies so insensibly, that it is evident that no other means, excepting
aqueous deposition, could have leveled the surface so equally. In the Maldiva atolls this is very conspicuous,
and likewise in some of the Caroline and Marshall Islands. In the former large spaces consist of sand and
SOFT CLAY; and Kotzebue speaks of clay having been found within one of the Marshall atolls. No doubt
this clay is calcareous mud, similar to that at Keeling Island, and to that at Bermuda already referred to, as
undistinguishable from disintegrated chalk, and which Lieutenant Nelson says is called there pipe−clay. (I
may here observe that on the coast of Brazil, where there is much coral, the soundings near the land are
described by Admiral Roussin, in the "Pilote du Bresil", as siliceous sand, mingled with much finely
comminuted particles of shells and coral. Further in the offing, for a space of 1,300 miles along the coast,
from the Abrolhos Islands to Maranham, the bottom in many places is composed of "tuf blanc, mele ou forme
de madrepores broyes." This white substance, probably, is analogous to that which occurs within the
above−mentioned lagoons; it is sometimes, according to Roussin, firm, and he compares it to mortar.)
Where the waves act with unequal force on the two sides of an atoll, the islets appear to be first formed, and
are generally of greater continuity on the more exposed shore. The islets, also, which are placed to leeward,
are in most parts of the Pacific liable to be occasionally swept entirely away by gales, equalling hurricanes in
violence, which blow in an opposite direction to the ordinary trade−wind. The absence of the islets on the
leeward side of atolls, or when present their lesser dimensions compared with those to windward, is a
comparatively unimportant fact; but in several instances the reef itself on the leeward side, retaining its usual
defined outline, does not rise to the surface by several fathoms. This is the case with the southern side of Peros
Banhos (Plate I., Figure 9) in the Chagos group, with Mourileu atoll (Frederick Lutke's "Voyage autour du
Monde," volume ii., page 291. See also his account of Namonouito, below, and the chart of Oulleay in the
Atlas.) in the Caroline Archipelago, and with the barrier−reef (Plate I., Figure of the Gambier Islands. I
allude to the latter reef, although belonging to another class, because Captain Beechey was first led by it to
observe the peculiarity in the question. At Peros Banhos the submerged part is nine miles in length, and lies at
an average depth of about five fathoms; its surface is nearly level, and consists of hard stone, with a thin
covering of loose sand. There is scarcely any living coral on it, even on the outer margin, as I have been
particularly assured by Captain Moresby; it is, in fact, a wall of dead coral−rock, having the same width and
transverse section with the reef in its ordinary state, of which it is a continuous portion. The living and perfect
parts terminate abruptly, and abut on the submerged portions, in the same manner as on the sides of an
ordinary passage through the reef. The reef to leeward in other cases is nearly or quite obliterated, and one
side of the lagoon is left open; for instance, at Oulleay (Caroline Archipelago), where a crescent−formed reef
is fronted by an irregular bank, on which the other half of the annular reef probably once stood. At
Namonouito, in the same Archipelago, both these modifications of the reef concur; it consists of a great flat
bank, with from twenty to twenty−five fathoms water on it; for a length of more than forty miles on its
southern side it is open and without any reef, whilst on the other sides it is bounded by a reef, in parts rising to
the surface and perfectly characterised, in parts lying some fathoms submerged. In the Chagos group there are
annular reefs, entirely submerged, which have the same structure as the submerged and defined portions just
described. The Speaker's Bank offers an excellent example of this structure; its central expanse, which is
about twenty−two fathoms deep, is twenty−four miles across; the external rim is of the usual width of annular
reefs, and is well−defined; it lies between six and eight fathoms beneath the surface, and at the same depth
there are scattered knolls in the lagoon. Captain Moresby believes the rim consists of dead rock, thinly
covered with sand, and he is certain this is the case with the external rim of the Great Chagos Bank, which is
also essentially a submerged atoll. In both these cases, as in the submerged portion of the reef at Peros
Banhos, Captain Moresby feels sure that the quantity of living coral, even on the outer edge overhanging the
deep−sea water, is quite insignificant. Lastly, in several parts of the Pacific and Indian Oceans there are banks,
lying at greater depths than in the cases just mentioned, of the same form and size with the neighbouring
atolls, but with their atoll−like structure wholly obliterated. It appears from the survey of Freycinet, that there
are banks of this kind in the Caroline Archipelago, and, as is reported, in the Low Archipelago. When we
discuss the origin of the different classes of coral formations, we shall see that the submerged state of the
whole of some atoll−formed reefs, and of portions of others, generally but not invariably on the leeward side,
and the existence of more deeply submerged banks now possessing little or no signs of their original atoll−like
structure, are probably the effects of a uniform cause,−−namely, the death of the coral, during the subsidence
of the area, in which the atolls or banks are situated.
There is seldom, with the exception of the Maldiva atolls, more than two or three channels, and generally only
one leading into the lagoon, of sufficient depth for a ship to enter. in small atolls, there is usually not even
one. Where there is deep water, for instance above twenty fathoms, in the middle of the lagoon, the channels
through the reef are seldom as deep as the centre,−−it may be said that the rim only of the saucer−shaped
hollow forming the lagoon is notched. Mr. Lyell ("Principles of Geology," volume iii., page 289.) has
observed that the growth of the coral would tend to obstruct all the channels through a reef, except those kept
open by discharging the water, which during high tide and the greater part of each ebb is thrown over its
circumference. Several facts indicate that a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed, during the change of the monsoon, the
sea discoloured to a distance off the entrances into the Maldiva and Chagos atolls. This, probably, would
check the growth of the coral in them, far more effectually than a mere current of water. In the many small
atolls without any channel, these causes have not prevented the entire ring attaining the surface. The channels,
like the submerged and effaced parts of the reef, very generally though not invariably occur on the leeward
side of the atoll, or on that side, according to Beechey (Beechey's "Voyage," 4to edition, volume i., page
189.), which, from running in the same direction with the prevalent wind, is not fully exposed to it. Passages
between the islets on the reef, through which boats can pass at high water, must not be confounded with
ship−channels, by which the annular reef itself is breached. The passages between the islets occur, of course,
on the windward as well as on the leeward side; but they are more frequent and broader to leeward, owing to
the lesser dimensions of the islets on that side.
At Keeling atoll the shores of the lagoon shelve gradually, where the bottom is of sediment, and irregularly or
abruptly where there are coral−reefs; but this is by no means the universal structure in other atolls. Chamisso
(Kotzebue's "First Voyage," volume iii., page 142.), speaking in general terms of the lagoons in the Marshall
atolls, says the lead generally sinks "from a depth of two or three fathoms to twenty or twenty−four, and you
may pursue a line in which on one side of the boat you may see the bottom, and on the other the azure−blue
deep water." The shores of the lagoon−like channel within the barrier−reef at Vanikoro have a similar
structure. Captain Beechey has described a modification of this structure (and he believes it is not uncommon)
in two atolls in the Low Archipelago, in which the shores of the lagoon descend by a few, broad, slightly
inclined ledges or steps: thus at Matilda atoll (Beechey's "Voyage," 4to edition, volume i, page 160. At
Whitsunday Island the bottom of the lagoon slopes gradually towards the centre, and then deepens suddenly,
the edge of the bank being nearly perpendicular. This bank is formed of coral and dead shells.), the great
exterior reef, the surface of which is gently inclined towards and beneath the surface of the lagoon, ends
abruptly in a little cliff three fathoms deep; at its foot, a ledge forty yards wide extends, shelving gently
inwards like the surface−reef, and terminated by a second little cliff five fathoms deep; beyond this, the
bottom of the lagoon slopes to twenty fathoms, which is the average depth of its centre. These ledges seem to
be formed of coral−rock; and Captain Beechey says that the lead often descended several fathoms through
holes in them. In some atolls, all the coral reefs or knolls in the lagoon come to the surface at low water; in
other cases of rarer occurrence, all lie at nearly the same depth beneath it, but most frequently they are quite
irregular,−−some with perpendicular, some with sloping sides,−−some rising to the surface, and others lying
at all intermediate depths from the bottom upwards. I cannot, therefore, suppose that the union of such reefs
could produce even one uniformly sloping ledge, and much less two or three, one beneath the other, and each
terminated by an abrupt wall. At Matilda Island, which offers the best example of the step−like structure,
Captain Beechey observes that the coral−knolls within the lagoon are quite irregular in their height. We shall
hereafter see that the theory which accounts for the ordinary form of atolls, apparently includes this occasional
peculiarity in their structure.
In the midst of a group of atolls, there sometimes occur small, flat, very low islands of coral formation, which
probably once included a lagoon, since filled up with sediment and coral−reefs. Captain Beechey entertains no
doubt that this has been the case with the two small islands, which alone of thirty−one surveyed by him in the
Low Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15 deg S.) is described by Chamisso
(Kotzebue's "First Voyage," volume iii., page 221.) as formed by a dam of madreporitic rock inclosing a flat
space, thinly covered with trees, into which the sea on the leeward side occasionally breaks. North Keeling
atoll appears to be in a rather less forward stage of conversion into land; it consists of a horse−shoe shaped
strip of land surrounding a muddy flat, one mile in its longest axis, which is covered by the sea only at high
water. When describing South Keeling atoll, I endeavoured to show how slow the final process of filling up a
lagoon must be; nevertheless, as all causes do tend to produce this effect, it is very remarkable that not one
instance, as I believe, is known of a moderately sized lagoon being filled up even to the low water−line at
spring−tides, much less of such a one being converted into land. It is, likewise, in some degree remarkable,
how few atolls, except small ones, are surrounded by a single linear strip of land, formed by the union of
separate islets. We cannot suppose that the many atolls in the Pacific and Indian Oceans all have had a late
origin, and yet should they remain at their present level, subjected only to the action of the sea and to the
growing powers of the coral, during as many centuries as must have elapsed since any of the earlier tertiary
epochs, it cannot, I think, be doubted that their lagoons and the islets on their reef, would present a totally
different appearance from what they now do. This consideration leads to the suspicion that some renovating
agency (namely subsidence) comes into play at intervals, and perpetuates their original structure.
(DESCRIPTION OF THE PLATES.
PLATE II.−−GREAT CHAGOS BANK, NEW CALEDONIA,MENCHIKOFF ATOLL, ETC.
FIGURE 1.−−GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey by Captain Moresby and
Lieutenant Powell; the parts which are shaded, with the exception of two or three islets on the western and
northern sides, do not rise to the surface, but are submerged from four to ten fathoms; the banks bounded by
the dotted lines lie from fifteen to twenty fathoms beneath the surface, and are formed of sand; the central
space is of mud, and from thirty to fifty fathoms deep.
FIGURE 2.−−A vertical section, on the same scale, in an eastern and western line across the Great Chagos
Bank, given for the sake of exhibiting more clearly its structure.
FIGURE 3.−−MENCHIKOFF ATOLL (or lagoon−island), in the Marshall Archipelago, Northern Pacific
Ocean; from Krusenstern's "Atlas of the Pacific;" originally surveyed by Captain Hagemeister; the depth
within the lagoons is unknown.
FIGURE 4.−−MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the Maldiva Archipelago;
from the survey by Captain Moresby and Lieutenant Powell; the white spaces in the middle of the separate
small reefs, both on the margin and in the middle part, are meant to represent little lagoons; but it was found
not possible to distinguish them clearly from the small islets, which have been formed on these same small
reefs; many of the smaller reefs could not be introduced; the nautical mark (dot over a dash) over the figures
250 and 200, between Mahlos Mahdoo and Horsburgh atoll and Powell's island, signifies that soundings were
not obtained at these depths.
FIGURE 5.−−NEW CALEDONIA, in the western part of the Pacific; from Krusenstern's "Atlas," compiled
from several surveys; I have slightly altered the northern point of the reef, in accordance with the "Atlas of the
Voyage of the 'Astrolabe'." In Krusenstern's "Atlas," the reef is represented by a single line with crosses; I
have for the sake of uniformity added an interior line.
FIGURE 6.−−MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by Captain Moresby and
Lieutenant Powell.)
SECTION 1.III.−−ATOLLS OF THE MALDIVA ARCHIPELAGO−−GREAT CHAGOS BANK.
Maldiva Archipelago.−−Ring−formed reefs, marginal and central.−−Great depths in the lagoons of the
southern atolls.−−Reefs in the lagoons all rising to the surface.−−Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the waves.−−Destruction of islets.−−Connection in the
position and submarine foundation of distinct atolls.−−The apparent disseverment of large atolls.−−The Great
Chagos Bank.−−Its submerged condition and extraordinary structure.
Although occasional references have been made to the Maldiva atolls, and to the banks in the Chagos group,
some points of their structure deserve further consideration. My description is derived from an examination of
the admirable charts lately published from the survey of Captain Moresby and Lieutenant Powell, and more
especially from information which Captain Moresby has communicated to me in the kindest manner.
The Maldiva Archipelago is 470 miles in length, with an average breadth of about 50 miles. The form and
dimensions of the atolls, and their singular position in a double line, may be seen, but not well, in the greatly
reduced chart (Figure 6) in Plate II. The dimensions of the longest atoll in the group (called by the double
name of Milla−dou−Madou and Tilla−dou−Matte) have already been given; it is 88 miles in a medial and
slightly curved line, and is less than 20 miles in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
across in one direction, and 34 in another, and the great included expanse of water has a depth of between 250
and 300 feet. The smaller atolls in this group differ in no respect from ordinary ones; but the larger ones are
remarkable from being breached by numerous deep−water channels leading into the lagoon; for instance,
there are 42 channels, through which a ship could enter the lagoon of Suadiva. In the three southern large
atolls, the separate portions of reef between these channels have the ordinary structure, and are linear; but in
the other atolls, especially the more northern ones, these portions are ring− formed, like miniature atolls.
Other ring−formed reefs rise out of the lagoons, in the place of those irregular ones which ordinarily occur
there. In the reduction of the chart of Mahlos Mahdoo (Plate II., Figure 4), it was not found easy to define the
islets and the little lagoons within each reef, so that the ring−formed structure is very imperfectly shown; in
the large published charts of Tilla−dou−Matte, the appearance of these rings, from standing further apart from
each other, is very remarkable. The rings on the margin are generally elongated; many of them are three, and
some even five miles, in diameter; those within the lagoon are usually smaller, few being more than two miles
across, and the greater number rather less than one. The depth of the little lagoon within these small annular
reefs is generally from five to seven fathoms, but occasionally more; and in Ari atoll many of the central ones
are twelve, and some even more than twelve fathoms deep. These rings rise abruptly from the platform or
bank, on which they are placed; their outer margin is invariably bordered by living coral (Captain Moresby
informs me that Millepora complanata is one of the commonest kinds on the outer margin, as it is at Keeling
atoll.) within which there is a flat surface of coral rock; of this flat, sand and fragments have in many cases
accumulated and been converted into islets, clothed with vegetation. I can, in fact, point out no essential
difference between these little ring−formed reefs (which, however, are larger, and contain deeper lagoons than
many true atolls that stand in the open sea), and the most perfectly characterised atolls, excepting that the
ring−formed reefs are based on a shallow foundation, instead of on the floor of the open sea, and that instead
of being scattered irregularly, they are grouped closely together on one large platform, with the marginal rings
arranged in a rudely formed circle.
The perfect series which can be traced from portions of simple linear reef, to others including long linear
lagoons, and from these again to oval or almost circular rings, renders it probable that the latter are merely
modifications of the linear or normal state. It is conformable with this view, that the ring−formed reefs on the
margin, even where most perfect and standing furthest apart, generally have their longest axes directed in the
line which the reef would have held, if the atoll had been bounded by an ordinary wall. We may also infer that
the central ring−formed reefs are modifications of those irregular ones, which are found in the lagoons of all
common atolls. It appears from the charts on a large scale, that the ring−like structure is contingent on the
marginal channels or breaches being wide; and, consequently, on the whole interior of the atoll being freely
exposed to the waters of the open sea. When the channels are narrow or few in number, although the lagoon
be of great size and depth (as in Suadiva), there are no ring−formed reefs; where the channels are somewhat
broader, the marginal portions of reef, and especially those close to the larger channels, are ring−formed, but
the central ones are not so; where they are broadest, almost every reef throughout the atoll is more or less
perfectly ring−formed. Although their presence is thus contingent on the openness of the marginal channels,
the theory of their formation, as we shall hereafter see, is included in that of the parent atolls, of which they
form the separate portions.
The lagoons of all the atolls in the southern part of the Archipelago are from ten to twenty fathoms deeper
than those in the northern part. This is well exemplified in the case of Addoo, the southernmost atoll in the
group, for although only nine miles in its longest diameter, it has a depth of thirty−nine fathoms, whereas all
the other small atolls have comparatively shallow lagoons; I can assign no adequate cause for this difference
in depth. In the central and deepest part of the lagoons, the bottom consists, as I am informed by Captain
Moresby, of stiff clay (probably a calcareous mud); nearer the border it consists of sand, and in the channels
through the reef, of hard sand−banks, sandstone, conglomerate rubble, and a little live coral. Close outside the
reef and the line joining its detached portions (where intersected by many channels), the bottom is sandy, and
it slopes abruptly into unfathomable depths. In most lagoons the depth is considerably greater in the centre
than in the channels; but in Tilla−dou−Matte, where the marginal ring−formed reefs stand far apart, the same
depth is carried across the entire atoll, from the deep−water line on one side to that on the other. I cannot
refrain from once again remarking on the singularity of these atolls,−−a great sandy and generally concave
disc rises abruptly from the unfathomable ocean, with its central expanse studded and its border symmetrically
fringed with oval basins of coral−rock, just lipping the surface of the sea, sometimes clothed with vegetation,
and each containing a little lake of clear water!
In the southern Maldiva atolls, of which there are nine large ones, all the small reefs within the lagoons come
to the surface, and are dry at low water spring−tides; hence in navigating them, there is no danger from
submarine banks. This circumstance is very remarkable, as within some atolls, for instance those of the
neighbouring Chagos group, not a single reef comes to the surface, and in most other cases a few only do, and
the rest lie at all intermediate depths from the bottom upwards. When treating of the growth of coral I shall
again refer to this subject.
Although in the neighbourhood of the Maldiva Archipelago the winds, during the monsoons, blow during
nearly an equal time from opposite quarters, and although, as I am informed by Captain Moresby, the westerly
winds are the strongest, yet the islets are almost all placed on the eastern side of the northern atolls, and on the
south−eastern side of the southern atolls. That the formation of the islets is due to detritus thrown up from the
outside, as in the ordinary manner, and not from the interior of the lagoons, may, I think be safely inferred
from several considerations, which it is hardly worth while to detail. As the easterly winds are not the
strongest, their action probably is aided by some prevailing swell or current.
In groups of atolls, exposed to a trade−wind, the ship−channels into the lagoons are almost invariably situated
on the leeward or less exposed side of the reef, and the reef itself is sometimes either wanting there, or is
submerged. A strictly analogous, but different fact, may be observed at the Maldiva atolls−−namely, that
where two atolls stand in front of each other, the breaches in the reef are the most numerous on their near, and
therefore less exposed, sides. Thus on the near sides of Ari and the two Nillandoo atolls, which face S. Male,
Phaleedoo, and Moloque atolls, there are seventy−three deep−water channels, and only twenty−five on their
outer sides; on the near side of the three latter named atolls there are fifty− six openings, and only
thirty−seven on their outsides. It is scarcely possible to attribute this difference to any other cause than the
somewhat different action of the sea on the two sides, which would ensue from the protection afforded by the
two rows of atolls to each other. I may here remark that in most cases, the conditions favourable to the greater
accumulation of fragments on the reef and to its more perfect continuity on one side of the atoll than on the
other, have concurred, but this has not been the case with the Maldivas; for we have seen that the islets are
placed on the eastern or south−eastern sides, whilst the breaches in the reef occur indifferently on any side,
where protected by an opposite atoll. The reef being more continuous on the outer and more exposed sides of
those atolls which stand near each other, accords with the fact, that the reef of the southern atolls is more
continuous than that of the northern ones; for the former, as I am informed by Captain Moresby, are more
constantly exposed than the northern atolls to a heavy surf.
The date of the first formation of some of the islets in this Archipelago is known to the inhabitants; on the
other hand, several islets, and even some of those which are believed to be very old, are now fast wearing
away. The work of destruction has, in some instances, been completed in ten years. Captain Moresby found
on one water−washed reef the marks of wells and graves, which were excavated when it supported an islet. In
South Nillandoo atoll, the natives say that three of the islets were formerly larger: in North Nillandoo there is
one now being washed away; and in this latter atoll Lieutenant Prentice found a reef, about six hundred yards
in diameter, which the natives positively affirmed was lately an island covered with cocoa−nut trees. It is now
only partially dry at low water spring−tides, and is (in Lieutenant Prentice's words) "entirely covered with live
coral and madrepore." In the northern part, also, of the Maldiva Archipelago and in the Chagos group, it is
known that some of the islets are disappearing. The natives attribute these effects to variations in the currents
of the sea. For my own part I cannot avoid suspecting that there must be some further cause, which gives rise
to such a cycle of change in the action of the currents of the great and open ocean.
Several of the atolls in this Archipelago are so related to each other in form and position, that at the first
glance one is led to suspect that they have originated in the disseverment of a single one. Male consists of
three perfectly characterised atolls, of which the shape and relative position are such, that a line drawn closely
round all three, gives a symmetrical figure; to see this clearly, a larger chart is required than that of the
Archipelago in Plate II.; the channel separating the two northern Male atolls is only little more than a mile
wide, and no bottom was found in it with 100 fathoms. Powell's Island is situated at the distance of two miles
and a half off the northern end of Mahlos Mahdoo (see Figure 4, Plate II.), at the exact point where the two
sides of the latter, if prolonged, would meet; no bottom, however, was found in the channel with 200 fathoms;
in the wider channel between Horsburgh atoll and the southern end of Mahlos Mahdoo, no bottom was found
with 250 fathoms. In these and similar cases, the relation consists only in the form and position of the atolls.
But in the channel between the two Nillandoo atolls, although three miles and a quarter wide, soundings were
struck at the depth of 200 fathoms; the channel between Ross and Ari atolls is four miles wide, and only 150
fathoms deep. Here then we have, besides the relation of form, a submarine connection. The fact of soundings
having been obtained between two separate and perfectly characterised atolls is in itself interesting, as it has
never, I believe, been effected in any of the many other groups of atolls in the Pacific and Indian seas. In
continuing to trace the connection of adjoining atolls, if a hasty glance be taken at the chart (Figure 4., Plate
II.) of Mahlos Mahdoo, and the line of unfathomable water be followed, no one will hesitate to consider it as
one atoll. But a second look will show that it is divided by a bifurcating channel, of which the northern arm is
about one mile and three−quarters in width, with an average depth of 125 fathoms, and the southern one
three−quarters of a mile wide, and rather less deep. These channels resemble in the slope of their sides and
general form, those which separate atolls in every respect distinct; and the northern arm is wider than that
dividing two of the Male atolls. The ring−formed reefs on the sides of this bifurcating channel are elongated,
so that the northern and southern portions of Mahlos Mahdoo may claim, as far as their external outline is
concerned, to be considered as distinct and perfect atolls. But the intermediate portion, lying in the fork of the
channel, is bordered by reefs less perfect than those which surround any other atoll in the group of equally
small dimensions. Mahlos Mahdoo, therefore, is in every respect in so intermediate a condition, that it may be
considered either as a single atoll nearly dissevered into three portions, or as three atolls almost perfect and
intimately connected. This is an instance of a very early stage of the apparent disseverment of an atoll, but a
still earlier one in many respects is exhibited at Tilla−dou− Matte. In one part of this atoll, the ring−formed
reefs stand so far apart from each other, that the inhabitants have given different names to the northern and
southern halves; nearly all the rings, moreover, are so perfect and stand so separate, and the space from which
they rise is so level and unlike a true lagoon, that we can easily imagine the conversion of this one great atoll,
not into two or three portions, but into a whole group of miniature atolls. A perfect series such as we have
here traced, impresses the mind with an idea of actual change; and it will hereafter be seen, that the theory of
subsidence, with the upward growth of the coral, modified by accidents of probable occurrence, will account
for the occasional disseverment of large atolls.
The Great Chagos bank alone remains to be described. In the Chagos group there are some ordinary atolls,
some annular reefs rising to the surface but without any islets on them, and some atoll−formed banks, either
quite submerged, or nearly so. Of the latter, the Great Chagos Bank is much the largest, and differs in its
structure from the others: a plan of it is given in Plate II., Figure 1, in which, for the sake of clearness, I have
had the parts under ten fathoms deep finely shaded: an east and west vertical section is given in Figure 2, in
which the vertical scale has been necessarily exaggerated. Its longest axis is ninety nautical miles, and another
line drawn at right angles to the first, across the broadest part, is seventy. The central part consists of a level
muddy flat, between forty and fifty fathoms deep, which is surrounded on all sides, with the exception of
some breaches, by the steep edges of a set of banks, rudely arranged in a circle. These banks consist of sand,
with a very little live coral; they vary in breadth from five to twelve miles, and on an average lie about sixteen
fathoms beneath the surface; they are bordered by the steep edges of a third narrow and upper bank, which
forms the rim to the whole. This rim is about a mile in width, and with the exception of two or three spots
where islets have been formed, is submerged between five and ten fathoms. It consists of smooth hard rock,
covered with a thin layer of sand, but with scarcely any live coral; it is steep on both sides, and outwards
slopes abruptly into unfathomable depths. At the distance of less than half a mile from one part, no bottom
was found with 190 fathoms; and off another point, at a somewhat greater distance, there was none with 210
fathoms. Small steep−sided banks or knolls, covered with luxuriantly growing coral, rise from the interior
expanse to the same level with the external rim, which, as we have seen, is formed only of dead rock. It is
impossible to look at the plan (Figure 1, Plate II.), although reduced to so small a scale, without at once
perceiving that the Great Chagos Bank is, in the words of Captain Moresby (This officer has had the kindness
to lend me an excellent MS. account of the Chagos Islands; from this paper, from the published charts, and
from verbal information communicated to me by Captain Moresby, the above account of the Great Chagos
Bank is taken.), "nothing more than a half−drowned atoll." But of what great dimensions, and of how
extraordinary an internal structure? We shall hereafter have to consider both the cause of its submerged
condition, a state common to other banks in the group, and the origin of the singular submarine terraces,
which bound the central expanse: these, I think, it can be shown, have resulted from a cause analogous to that
which has produced the bifurcating channel across Mahlos Mahdoo.

Chapter II

Barrier reefs

Closely resemble in general form and structure atoll−reefs.−−Width and depth of the
lagoon−channels.−−Breaches through the reef in front of valleys, and generally on the leeward side.−−Checks
to the filling up of the lagoon−channels.−−Size and constitution of the encircled islands.−− Number of islands
within the same reef.−−Barrier−reefs of New Caledonia and Australia.−−Position of the reef relative to the
slope of the adjoining land.−−Probable great thickness of barrier−reefs.
The term "barrier" has been generally applied to that vast reef which fronts the N.E. shore of Australia, and by
most voyagers likewise to that on the western coast of New Caledonia. At one time I thought it convenient
thus to restrict the term, but as these reefs are similar in structure, and in position relatively to the land, to
those, which, like a wall with a deep moat within, encircle many smaller islands, I have classed them together.
The reef, also, on the west coast of New Caledonia, circling round the extremities of the island, is an
intermediate form between a small encircling reef and the Australian barrier, which stretches for a thousand
miles in nearly a straight line.
The geographer Balbi has in effect described those barrier−reefs, which encircle moderately sized islands, by
calling them atolls with high land rising from within their central expanse. The general resemblance between
the reefs of the barrier and atoll classes may be seen in the small, but accurately reduced charts on Plate I.
(The authorities from which these charts have been reduced, together with some remarks on them and
descriptive of the Plates, are given separately.), and this resemblance can be further shown to extend to every
part of the structure. Beginning with the outside of the reef; many scattered soundings off Gambier, Oualan,
and some other encircled islands, show that close to the breakers there exists a narrow shelving margin,
beyond which the ocean becomes suddenly unfathomable; but off the west coast of New Caledonia, Captain
Kent (Dalrymple, "Hydrog. Mem." volume iii.) found no bottom with 150 fathoms, at two ships' length from
the reef; so that the slope here must be nearly as precipitous as off the Maldiva atolls.
I can give little information regarding the kinds of corals which live on the outer margin. When I visited the
reef at Tahiti, although it was low water, the surf was too violent for me to see the living masses; but,
according to what I heard from some intelligent native chiefs, they resemble in their rounded and branchless
forms, those on the margin of Keeling atoll. The extreme verge of the reef, which was visible between the
breaking waves at low water, consisted of a rounded, convex, artificial−like breakwater, entirely coated with
Nulliporae, and absolutely similar to that which I have described at Keeling atoll. From what I heard when at
Tahiti, and from the writings of the Revs. W. Ellis and J. Williams, I conclude that this peculiar structure is
common to most of the encircled islands of the Society Archipelago. The reef within this mound or
breakwater, has an extremely irregular surface, even more so than between the islets on the reef of Keeling
atoll, with which alone (as there are no islets on the reef of Tahiti) it can properly be compared. At Tahiti, the
reef is very irregular in width; but round many other encircled islands, for instance, Vanikoro or Gambier
Islands (Figures 1 and 8, Plate I.), it is quite as regular, and of the same average width, as in true atolls. Most
barrier−reefs on the inner side slope irregularly into the lagoon−channel (as the space of deep water separating
the reef from the included land may be called), but at Vanikoro the reef slopes only for a short distance, and
then terminates abruptly in a submarine wall, forty feet high,−−a structure absolutely similar to that described
by Chamisso in the Marshall atolls.
In the Society Archipelago, Ellis (Consult, on this and other points, the "Polynesian Researches," by the Rev.
W. Ellis, an admirable work, full of curious information.) states, that the reefs generally lie at the distance of
from one to one and a half miles, and, occasionally, even at more than three miles, from the shore. The central
mountains are generally bordered by a fringe of flat, and often marshy, alluvial land, from one to four miles in
width. This fringe consists of coral−sand and detritus thrown up from the lagoon−channel, and of soil washed
down from the hills; it is an encroachment on the channel, analogous to that low and inner part of the islets in
many atolls which is formed by the accumulation of matter from the lagoon. At Hogoleu (Figure 2, Plate I.),
in the Caroline Archipelago (See "Hydrographical Mem." and the "Atlas of the Voyage of the 'Astrolabe'," by
Captain Dumont D'Urville, page 428.), the reef on the south side is no less than twenty miles; on the east side,
five; and on the north side, fourteen miles from the encircled high islands.
The lagoon channels may be compared in every respect with true lagoons. In some cases they are open, with a
level bottom of fine sand; in others they are choked up with reefs of delicately branched corals, which have
the same general character as those within the Keeling atoll. These internal reefs either stand separately, or
more commonly skirt the shores of the included high islands. The depth of the lagoon−channel round the
Society Islands varies from two or three to thirty fathoms; in Cook's (See the chart in volume i. of
Hawkesworth's 4to edition of "Cook's First Voyage.") chart of Ulieta, however, there is one sounding laid
down of forty−eight fathoms; at Vanikoro there are several of fifty−four and one of fifty−six and a half
fathoms (English), a depth which even exceeds by a little that of the interior of the great Maldiva atolls. Some
barrier−reefs have very few islets on them; whilst others are surmounted by numerous ones; and those round
part of Bolabola (Plate I., Figure 5) form a single linear strip. The islets first appear either on the angles of the
reef, or on the sides of the breaches through it, and are generally most numerous on the windward side. The
reef to leeward retaining its usual width, sometimes lies submerged several fathoms beneath the surface; I
have already mentioned Gambier Island as an instance of this structure. Submerged reefs, having a less
defined outline, dead, and covered with sand, have been observed (see Appendix) off some parts of Huaheine
and Tahiti. The reef is more frequently breached to leeward than to windward; thus I find in Krusenstern's
"Memoir on the Pacific," that there are passages through the encircling reef on the leeward side of each of the
seven Society Islands, which possess ship−harbours; but that there are openings to windward through the reef
of only three of them. The breaches in the reef are seldom as deep as the interior lagoon−like channel; they
generally occur in front of the main valleys, a circumstance which can be accounted for, as will be seen in the
fourth chapter, without much difficulty. The breaches being situated in front of the valleys, which descend
indifferently on all sides, explains their more frequent occurrence through the windward side of barrier−reefs
than through the windward side of atolls,−−for in atolls there is no included land to influence the position of
the breaches.
It is remarkable, that the lagoon−channels round mountainous islands have not in every instance been long
ago filled up with coral and sediment; but it is more easily accounted for than appears at first sight. In cases
like that of Hogoleu and the Gambier Islands, where a few small peaks rise out of a great lagoon, the
conditions scarcely differ from those of an atoll, and I have already shown, at some length, that the filling up
of a true lagoon must be an extremely slow process. Where the channel is narrow, the agency, which on
unprotected coasts is most productive of sediment, namely the force of the breakers, is here entirely excluded,
and the reef being breached in the front of the main valleys, much of the finer mud from the rivers must be
transported into the open sea. As a current is formed by the water thrown over the edge of atoll−formed reefs,
which carries sediment with it through the deep−water breaches, the same thing probably takes place in
barrier−reefs, and this would greatly aid in preventing the lagoon−channel from being filled up. The low
alluvial border, however, at the foot of the encircled mountains, shows that the work of filling up is in
progress; and at Maura (Plate I., Figure 6), in the Society group, it has been almost effected, so that there
remains only one harbour for small craft.
If we look at a set of charts of barrier−reefs, and leave out in imagination the encircled land, we shall find
that, besides the many points already noticed of resemblance, or rather of identity in structure with atolls,
there is a close general agreement in form, average dimensions, and grouping. Encircling barrier−reefs, like
atolls, are generally elongated, with an irregularly rounded, though sometimes angular outline. There are atolls
of all sizes, from less than two miles in diameter to sixty miles (excluding Tilla−dou−Matte, as it consists of a
number of almost independent atoll−formed reefs); and there are encircling barrier−reefs from three miles and
a half to forty−six miles in diameter,−−Turtle Island being an instance of the former, and Hogoleu of the
latter. At Tahiti the encircled island is thirty−six miles in its longest axis, whilst at Maurua it is only a little
more than two miles. It will be shown, in the last chapter in this volume, that there is the strictest resemblance
in the grouping of atolls and of common islands, and consequently there must be the same resemblance in the
grouping of atolls and of encircling barrier−reefs.
The islands lying within reefs of this class, are of very various heights. Tahiti is 7,000 feet (The height of
Tahiti is given from Captain Beechey; Maurua from Mr. F.D. Bennett ("Geograph. Journ." volume viii., page
220); Aitutaki from measurements made on board the "Beagle"; and Manouai or Harvey Island, from an
estimate by the Rev. J. Williams. The two latter islands, however, are not in some respects well characterised
examples of the encircled class.); Maurua about 800; Aitutaki 360, and Manouai only 50. The geological
nature of the included land varies: in most cases it is of ancient volcanic origin, owing apparently to the fact
that islands of this nature are most frequent within all great seas; some, however, are of madreporitic
limestone, and others of primary formation, of which latter kind New Caledonia offers the best example. The
central land consists either of one island, or of several: thus, in the Society group, Eimeo stands by itself;
while Taha and Raiatea (Figure 3, Plate I.), both moderately large islands of nearly equal size, are included in
one reef. Within the reef of the Gambier group there are four large and some smaller islands (Figure 8, Plate
I.); within that of Hogoleu (Figure 2, Plate I.) nearly a dozen small islands are scattered over the expanse of
one vast lagoon.
After the details now given, it may be asserted that there is not one point of essential difference between
encircling barrier−reefs and atolls: the latter enclose a simple sheet of water, the former encircle an expanse
with one or more islands rising from it. I was much struck with this fact, when viewing, from the heights of
Tahiti, the distant island of Eimeo standing within smooth water, and encircled by a ring of snow−white
breakers. Remove the central land, and an annular reef like that of an atoll in an early stage of its formation is
left; remove it from Bolabola, and there remains a circle of linear coral−islets, crowned with tall cocoa−nut
trees, like one of the many atolls scattered over the Pacific and Indian Oceans.
The barrier−reefs of Australia and of New Caledonia deserve a separate notice from their great dimensions.
The reef on the west coast of New Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a length of
many leagues it seldom approaches within eight miles of the shore; and near the southern end of the island,
the space between the reef and the land is sixteen miles in width. The Australian barrier extends, with a few
interruptions, for nearly a thousand miles; its average distance from the land is between twenty and thirty
miles; and in some parts from fifty to seventy. The great arm of the sea thus included, is from ten to
twenty−five fathoms deep, with a sandy bottom; but towards the southern end, where the reef is further from
the shore, the depth gradually increases to forty, and in some parts to more than sixty fathoms. Flinders
(Flinders' "Voyage to Terra Australis," volume ii., page 88.) has described the surface of this reef as
consisting of a hard white agglomerate of different kinds of coral, with rough projecting points. The outer
edge is the highest part; it is traversed by narrow gullies, and at rare intervals is breached by ship−channels.
The sea close outside is profoundly deep; but, in front of the main breaches, soundings can sometimes be
obtained. Some low islets have been formed on the reef.
(PLATE: UNNAMED, THREE VERTICAL SECTIONS (WOODCUT DIAGRAMS):
1. VANIKORO, from the "Atlas of the Voyage of the 'Astrolabe'," by D. D'Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the "Atlas of the Voyage of the 'Coquille'," by Duperrey.
The horizontal line is the level of the sea, from which on the right hand a plummet descends, representing a
depth of 200 fathoms, or 1,200 feet. The vertical shading shows the section of the land, and the horizontal
shading that of the encircling barrier−reef: from the smallness of the scale, the lagoon−channel could not be
represented.
AA.−−Outer edge of the coral−reefs, where the sea breaks.
BB.−−The shore of the encircled islands.)
There is one important point in the structure of barrier−reefs which must here be considered. The
accompanying diagrams represent north and south vertical sections, taken through the highest points of
Vanikoro, Gambier, and Maurua Islands, and through their encircling reefs. The scale both in the horizontal
and vertical direction is the same, namely, a quarter of an inch to a nautical mile. The height and width of
these islands is known; and I have attempted to represent the form of the land from the shading of the hills in
the large published charts. It has long been remarked, even from the time of Dampier, that considerable degree
of relation subsists between the inclination of that part of the land which is beneath water and that above it;
hence the dotted line in the three sections, probably, does not widely differ in inclination from the actual
submarine prolongation of the land. If we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must conclude that the vertical thickness of
these barrier coral−reefs is very great.
I must observe that if the sections had been taken in any other direction across these islands, or across other
encircled islands (In the fifth chapter an east and west section across the Island of Bolabola and its
barrier−reefs is given, for the sake of illustrating another point. The unbroken line in it (woodcut No. 5) is the
section referred to. The scale is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of the
'Coquille'," by Duperrey. The depth of the lagoon−channel is exaggerated.), the result would have been the
same. In the succeeding chapter it will be shown that reef−building polypifers cannot flourish at great
depths,−−for instance, it is highly improbable that they could exist at a quarter of the depth represented by the
plummet on the right hand of the woodcut. Here there is a great APPARENT difficulty−−how were the basal
parts of these barrier−reef formed? It will, perhaps, occur to some, that the actual reefs formed of coral are not
of great thickness, but that before their first growth, the coasts of these encircled islands were deeply eaten
into, and a broad but shallow submarine ledge thus left, on the edge of which the coral grew; but if this had
been the case, the shore would have been invariably bounded by lofty cliffs, and not have sloped down to the
lagoon−channel, as it does in many instances. On this view (The Rev. D. Tyerman and Mr. Bennett ("Journal
of Voyage and Travels," volume i., page 215) have briefly suggested this explanation of the origin of the
encircling reefs of the Society Islands.), moreover, the cause of the reef springing up at such a great distance
from the land, leaving a deep and broad moat within, remains altogether unexplained. A supposition of the
same nature, and appearing at first more probable is, that the reefs sprung up from banks of sediment, which
had accumulated round the shore previously to the growth of the coral; but the extension of a bank to the same
distance round an unbroken coast, and in front of those deep arms of the sea (as in Raiatea, see Plate II.,
Figure 3) which penetrate nearly to the heart of some encircled islands, is exceedingly improbable. And why,
again, should the reef spring up, in some cases steep on both sides like a wall, at a distance of two, three or
more miles from the shore, leaving a channel often between two hundred and three hundred feet deep, and
rising from a depth which we have reason to believe is destructive to the growth of coral? An admission of
this nature cannot possibly be made. The existence, also, of the deep channel, utterly precludes the idea of the
reef having grown outwards, on a foundation slowly formed on its outside, by the accumulation of sediment
and coral detritus. Nor, again, can it be asserted, that the reef−building corals will not grow, excepting at a
great distance from the land; for, as we shall soon see, there is a whole class of reefs, which take their name
from growing closely attached (especially where the sea is deep) to the beach. At New Caledonia (see Plate
II., Figure 5) the reefs which run in front of the west coast are prolonged in the same line 150 miles beyond
the northern extremity of the island, and this shows that some explanation, quite different from any of those
just suggested, is required. The continuation of the reefs on each side of the submarine prolongation of New
Caledonia, is an exceedingly interesting fact, if this part formerly existed as the northern extremity of the
island, and before the attachment of the coral had been worn down by the action of the sea, or if it originally
existed at its present height, with or without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but fronting its sides, in the same line with the
reefs which front the shores of the lofty island? We shall hereafter see, that there is one, and I believe only
AA.−−Outer edge of the coral−reefs, where the sea breaks.
BB.−−The shore of the encircled islands.)
There is one important point in the structure of barrier−reefs which must here be considered. The
accompanying diagrams represent north and south vertical sections, taken through the highest points of
Vanikoro, Gambier, and Maurua Islands, and through their encircling reefs. The scale both in the horizontal
and vertical direction is the same, namely, a quarter of an inch to a nautical mile. The height and width of
these islands is known; and I have attempted to represent the form of the land from the shading of the hills in
the large published charts. It has long been remarked, even from the time of Dampier, that considerable degree
of relation subsists between the inclination of that part of the land which is beneath water and that above it;
hence the dotted line in the three sections, probably, does not widely differ in inclination from the actual
submarine prolongation of the land. If we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must conclude that the vertical thickness of
these barrier coral−reefs is very great.
I must observe that if the sections had been taken in any other direction across these islands, or across other
encircled islands (In the fifth chapter an east and west section across the Island of Bolabola and its
barrier−reefs is given, for the sake of illustrating another point. The unbroken line in it (woodcut No. 5) is the
section referred to. The scale is .57 of an inch to a mile; it is taken from the "Atlas of the Voyage of the
'Coquille'," by Duperrey. The depth of the lagoon−channel is exaggerated.), the result would have been the
same. In the succeeding chapter it will be shown that reef−building polypifers cannot flourish at great
depths,−−for instance, it is highly improbable that they could exist at a quarter of the depth represented by the
plummet on the right hand of the woodcut. Here there is a great APPARENT difficulty−−how were the basal
parts of these barrier−reef formed? It will, perhaps, occur to some, that the actual reefs formed of coral are not
of great thickness, but that before their first growth, the coasts of these encircled islands were deeply eaten
into, and a broad but shallow submarine ledge thus left, on the edge of which the coral grew; but if this had
been the case, the shore would have been invariably bounded by lofty cliffs, and not have sloped down to the
lagoon−channel, as it does in many instances. On this view (The Rev. D. Tyerman and Mr. Bennett ("Journal
of Voyage and Travels," volume i., page 215) have briefly suggested this explanation of the origin of the
encircling reefs of the Society Islands.), moreover, the cause of the reef springing up at such a great distance
from the land, leaving a deep and broad moat within, remains altogether unexplained. A supposition of the
same nature, and appearing at first more probable is, that the reefs sprung up from banks of sediment, which
had accumulated round the shore previously to the growth of the coral; but the extension of a bank to the same
distance round an unbroken coast, and in front of those deep arms of the sea (as in Raiatea, see Plate II.,
Figure 3) which penetrate nearly to the heart of some encircled islands, is exceedingly improbable. And why,
again, should the reef spring up, in some cases steep on both sides like a wall, at a distance of two, three or
more miles from the shore, leaving a channel often between two hundred and three hundred feet deep, and
rising from a depth which we have reason to believe is destructive to the growth of coral? An admission of
this nature cannot possibly be made. The existence, also, of the deep channel, utterly precludes the idea of the
reef having grown outwards, on a foundation slowly formed on its outside, by the accumulation of sediment
and coral detritus. Nor, again, can it be asserted, that the reef−building corals will not grow, excepting at a
great distance from the land; for, as we shall soon see, there is a whole class of reefs, which take their name
from growing closely attached (especially where the sea is deep) to the beach. At New Caledonia (see Plate
II., Figure 5) the reefs which run in front of the west coast are prolonged in the same line 150 miles beyond
the northern extremity of the island, and this shows that some explanation, quite different from any of those
just suggested, is required. The continuation of the reefs on each side of the submarine prolongation of New
Caledonia, is an exceedingly interesting fact, if this part formerly existed as the northern extremity of the
island, and before the attachment of the coral had been worn down by the action of the sea, or if it originally
existed at its present height, with or without beds of sediment on each flank, how can we possibly account for
the reefs, not growing on the crest of this submarine portion, but fronting its sides, in the same line with the
reefs which front the shores of the lofty island? We shall hereafter see, that there is one, and I believe only

one, solution of this difficulty.
One other supposition to account for the position of encircling barrier−reefs remains, but it is almost too
preposterous to be mentioned; namely, that they rest on enormous submarine craters, surrounding the included
islands. When the size, height, and form of the islands in the Society group are considered, together with the
fact that all are thus encircled, such a notion will be rejected by almost every one. New Caledonia, moreover,
besides its size, is composed of primitive formations, as are some of the Comoro Islands (I have been
informed that this is the case by Dr. Allan of Forres, who has visited this group.); and Aitutaki consists of
calcareous rock. We must, therefore, reject these several explanations, and conclude that the vertical thickness
of barrier−reefs, from their outer edges to the foundation on which they rest (from AA in the section to the
dotted lines) is really great; but in this, there is no difficulty, for it is not necessary to suppose that the coral
has sprung up from an immense depth, as will be evident when the theory of the upward growth of
coral−reefs, during the slow subsidence of their foundation, is discussed.

Chapter III

Fringing or shore - reefs

Reefs of Mauritius.−−Shallow channel within the reef.−−Its slow filling up.−−Currents of water formed
within it.−−Upraised reefs.−−Narrow fringing−reefs in deep seas.−−Reefs on the coast of East Africa and of
Brazil.−−Fringing−reefs in very shallow seas, round banks of sediment and on worn−down
islands.−−Fringing−reefs affected by currents of the sea.−− Coral coating the bottom of the sea, but not
forming reefs.
Fringing−reefs, or, as they have been called by some voyagers, shore−reefs, whether skirting an island or part
of a continent, might at first be thought to differ little, except in generally being of less breadth, from
barrier−reefs. As far as the superficies of the actual reef is concerned this is the case; but the absence of an
interior deep−water channel, and the close relation in their horizontal extension with the probable slope
beneath the sea of the adjoining land, present essential points of difference.
The reefs which fringe the island of Mauritius offer a good example of this class. They extend round its whole
circumference, with the exception of two or three parts (This fact is stated on the authority of the Officier du
Roi, in his extremely interesting "Voyage a l'Isle de France," undertaken in 1768. According to Captain
Carmichael (Hooker's "Bot. Misc." volume ii., page 316) on one part of the coast there is a space for sixteen
miles without a reef.), where the coast is almost precipitous, and where, if as is probable the bottom of the sea
has a similar inclination, the coral would have no foundation on which to become attached. A similar fact may
sometimes be observed even in reefs of the barrier class, which follow much less closely the outline of the
adjoining land; as, for instance, on the south−east and precipitous side of Tahiti, where the encircling reef is
interrupted. On the western side of the Mauritius, which was the only part I visited, the reef generally lies at
the distance of about half a mile from the shore; but in some parts it is distant from one to two, and even three
miles. But even in this last case, as the coast−land is gently inclined from the foot of the mountains to the
sea−beach, and as the soundings outside the reef indicate an equally gentle slope beneath the water, there is no
reason for supposing that the basis of the reef, formed by the prolongation of the strata of the island, lies at a
greater depth than that at which the polypifers could begin constructing the reef. Some allowance, however,
must be made for the outward extension of the corals on a foundation of sand and detritus, formed from their
own wear, which would give to the reef a somewhat greater vertical thickness, than would otherwise be
possible.
The outer edge of the reef on the western or leeward side of the island is tolerably well defined, and is a little
higher than any other part. It chiefly consists of large strongly branched corals, of the genus Madrepora, which
also form a sloping bed some way out to sea: the kinds of coral growing in this part will be described in the
ensuing chapter. Between the outer margin and the beach, there is a flat space with a sandy bottom and a few
tufts of living coral; in some parts it is so shallow, that people, by avoiding the deeper holes and gullies, can
wade across it at low water; in other parts it is deeper, seldom however exceeding ten or twelve feet, so that it
offers a safe coasting channel for boats. On the eastern and windward side of the island, which is exposed to a
heavy surf, the reef was described to me as having a hard smooth surface, very slightly inclined inwards, just
covered at low−water, and traversed by gullies; it appears to be quite similar in structure to the reefs of the
barrier and atoll classes.
The reef of Mauritius, in front of every river and streamlet, is breached by a straight passage: at Grand Port,
however, there is a channel like that within a barrier−reef; it extends parallel to the shore for four miles, and
has an average depth of ten or twelve fathoms; its presence may probably be accounted for by two rivers
which enter at each end of the channel, and bend towards each other. The fact of reefs of the fringing class
being always breached in front of streams, even of those which are dry during the greater part of the year, will
be explained, when the conditions unfavourable to the growth of coral are considered. Low coral−islets, like
those on barrier−reefs and atolls, are seldom formed on reefs of this class, owing apparently in some cases to
their narrowness, and in others to the gentle slope of the reef outside not yielding many fragments to the
breakers. On the windward side, however, of the Mauritius, two or three small islets have been formed.
It appears, as will be shown in the ensuing chapter, that the action of the surf is favourable to the vigorous
growth of the stronger corals, and that sand or sediment, if agitated by the waves, is injurious to them. Hence
it is probable that a reef on a shelving shore, like that of Mauritius, would at first grow up, not attached to the
actual beach, but at some little distance from it; and the corals on the outer margin would be the most
vigorous. A shallow channel would thus be formed within the reef, and as the breakers are prevented acting on
the shores of the island, and as they do not ordinarily tear up many fragments from the outside, and as every
streamlet has its bed prolonged in a straight line through the reef, this channel could be filled up only very
slowly with sediment. But a beach of sand and of fragments of the smaller kinds of coral seems, in the case of
Mauritius, to be slowly encroaching on the shallow channel. On many shelving and sandy coasts, the breakers
tend to form a bar of sand a little way from the beach, with a slight increase of depth within it; for instance,
Captain Grey (Captain Grey's "Journal of Two Expeditions," volume i. page 369.) states that the west coast of
Australia, in latitude 24 deg., is fronted by a sand bar about two hundred yards in width, on which there is
only two feet of water; but within it the depth increases to two fathoms. Similar bars, more or less perfect,
occur on other coasts. In these cases I suspect that the shallow channel (which no doubt during storms is
occasionally obliterated) is scooped out by the flowing away of the water thrown beyond the line, on which
the waves break with the greatest force. At Pernambuco a bar of hard sandstone (I have described this singular
structure in the "London and Edinburgh Phil. Mag." October 1841.), which has the same external form and
height as a coral−reef, extends nearly parallel to the coast; within this bar currents, apparently caused by the
water thrown over it during the greater part of each tide, run strongly, and are wearing away its inner wall.
From these facts it can hardly be doubted, that within most fringing−reefs, especially within those lying some
distance from the land, a return stream must carry away the water thrown over the outer edge; and the current
thus produced, would tend to prevent the channel being filled up with sediment, and might even deepen it
under certain circumstances. To this latter belief I am led, by finding that channels are almost universally
present within the fringing−reefs of those islands which have undergone recent elevatory movements; and this
could hardly have been the case, if the conversion of the very shallow channel into land had not been
counteracted to a certain extent.
A fringing−reef, if elevated in a perfect condition above the level of the sea, ought to present the singular
appearance of a broad dry moat within a low mound. The author ("Voyage a l'Isle de France, par un Officier
du Roi," part i., pages 192, 200.) of an interesting pedestrian tour round the Mauritius, seems to have met with
a structure of this kind: he says "J'observai que la, ou la mer etale, independamment des rescifs du large, il y a
terre UNE ESPECE D'EFFONCEMENT ou chemin couvert naturel. On y pourrait mettre du canon," etc. In
another place he adds, "Avant de passer le Cap, on remarque un gros banc de corail eleve de plus de quinze
pieds: c'est une espece de rescif, que la mer abandonne, il regne au pied une longue flaque d'eau, dont on
pourrait faire un bassin pour de petits vaisseaux." But the margin of the reef, although the highest and most
perfect part, from being most exposed to the surf, would generally during a slow rise of the land be either
partially or entirely worn down to that level, at which corals could renew their growth on its upper edge. On
some parts of the coast−land of Mauritius there are little hillocks of coral−rock, which are either the last
remnants of a continuous reef, or of low islets formed on it. I observed that two such hillocks between
Tamarin Bay and the Great Black River; they were nearly twenty feet high, about two hundred yards from the
present beach, and about thirty feet above its level. They rose abruptly from a smooth surface, strewed with
worn fragments of coral. They consisted in their lower part of hard calcareous sandstone, and in their upper of
great blocks of several species of Astraea and Madrepora, loosely aggregated; they were divided into irregular
beds, dipping seaward, in one hillock at an angle of 8 deg., and in the other at 18 deg. I suspect that the
superficial parts of the reefs, which have been upraised together with the islands they fringe, have generally
been much more modified by the wearing action of the sea, than those of Mauritius.
Many islands are fringed by reefs quite similar to those of Mauritius (I may give Cuba, as another instance;
Mr. Taylor ("Loudon's Mag. of Nat. Hist." volume ix., page 449) has described a reef several miles in length
between Gibara and Vjaro, which extends parallel to the shore at the distance of between half and the third
part of a mile, and encloses a space of shallow water, with a sandy bottom and tufts of coral. Outside the edge
of the reef, which is formed of great branching corals, the depth is six and seven fathoms. This coast has been
upheaved at no very distant geological period."); but on coasts where the sea deepens very suddenly the reefs
are much narrower, and their limited extension seems evidently to depend on the high inclination of the
submarine slope; a relation, which, as we have seen, does not exist in reefs of the barrier class. The
fringing−reefs on steep coasts are frequently not more than from fifty to one hundred yards in width; they
have a nearly smooth, hard surface, scarcely uncovered at low water, and without any interior shoal channel,
like that within those fringing−reefs, which lie at a greater distance from the land. The fragments torn up
during gales from the outer margin are thrown over the reef on the shores of the island. I may give as
instances, Wateeo, where the reef is described by Cook as being a hundred yards wide; and Mauti and
Elizabeth Islands (Mauti is described by Lord Byron in the voyage of H.M.S. "Blonde", and Elizabeth Island
by Captain Beechey.), where it is only fifty yards in width: the sea round these islands is very deep.
Fringing−reefs, like barrier−reefs, both surround islands, and front the shores of continents. In the charts of
the eastern coast of Africa, by Captain Owen, many extensive fringing−reefs are laid down; thus, for a space
of nearly forty miles, from latitude 1 deg 15' to 1 deg 45' S., a reef fringes the shore at an average distance of
rather more than one mile, and therefore at a greater distance than is usual in reefs of this class; but as the
coast−land is not lofty, and as the bottom shoals very gradually (the depth being only from eight to fourteen
fathoms at a mile and a half outside the reef), its extension thus far from the land offers no difficulty. The
external margin of this reef is described, as formed of projecting points, within which there is a space, from
six to twelve feet deep, with patches of living coral on it. At Mukdeesha (latitude 2 deg 1' N.) "the port is
formed," it is said (Owen's "Africa," volume i., page 357, from which work the foregoing facts are likewise
taken.) "by a long reef extending eastward, four or five miles, within which there is a narrow channel, with ten
to twelve feet of water at low spring−tides;" it lies at the distance of a quarter of a mile from the shore. Again,
in the plan of Mombas (latitude 4 deg S.), a reef extends for thirty−six miles, at the distance of from half a
mile to one mile and a quarter from the shore; within it, there is a channel navigable "for canoes and small
craft," between six and fifteen feet deep: outside the reef the depth is about thirty fathoms at the distance of
nearly half a mile. Part of this reef is very symmetrical, and has a uniform breadth of two hundred yards.
The coast of Brazil is in many parts fringed by reefs. Of these, some are not of coral formation; for instance,
those near Bahia and in front of Pernambuco; but a few miles south of this latter city, the reef follows (See
Baron Roussin's "Pilote du Bresil," and accompanying hydrographical memoir.) so closely every turn of the
shore, that I can hardly doubt it is of coral; it runs at the distance of three−quarters of a mile from the land,
and within it the depth is from ten to fifteen feet. I was assured by an intelligent pilot that at Ports Frances and
Maceio, the outer part of the reef consists of living coral, and the inner of a white stone, full of large irregular
cavities, communicating with the sea. The bottom of the sea off the coast of Brazil shoals gradually to
between thirty and forty fathoms, at the distance of between nine and ten leagues from the land.
From the description now given, we must conclude that the dimensions and structure of fringing−reefs depend
entirely on the greater or less inclination of the submarine slope, conjoined with the fact that reef−building
polypifers can exist only at limited depths. It follows from this, that where the sea is very shallow, as in the
Persian Gulf and in parts of the East Indian Archipelago, the reefs lose their fringing character, and appear as
separate and irregularly scattered patches, often of considerable area. From the more vigorous growth of the
coral on the outside, and from the conditions being less favourable in several respects within, such reefs are
generally higher and more perfect in their marginal than in their central parts; hence these reefs sometimes
assume (and this circumstance ought not to be overlooked) the appearance of atolls; but they differ from atolls
in their central expanse being much less deep, in their form being less defined, and in being based on a
shallow foundation. But when in a deep sea reefs fringe banks of sediment, which have accumulated beneath
the surface, round either islands or submerged rocks, they are distinguished with difficulty on the one hand
from encircling barrier−reefs, and on the other from atolls. In the West Indies there are reefs, which I should
probably have arranged under both these classes, had not the existence of large and level banks, lying a little
beneath the surface, ready to serve as the basis for the attachment of coral, been occasionally brought into
view by the entire or partial absence of reefs on them, and had not the formation of such banks, through the
accumulation of sediment now in progress, been sufficiently evident. Fringing−reefs sometimes coat, and thus
protect the foundations of islands, which have been worn down by the surf to the level of the sea. According
to Ehrenberg, this has been extensively the case with the islands in the Red Sea, which formerly ranged
parallel to the shores of the mainland, with deep water within them: hence the reefs now coating their bases
are situated relatively to the land like barrier−reefs, although not belonging to that class; but there are, as I
believe, in the Red Sea some true barrier−reefs. The reefs of this sea and of the West Indies will be described
in the Appendix. In some cases, fringing−reefs appear to be considerably modified in outline by the course of
the prevailing currents. Dr. J. Allan informs me that on the east coast of Madagascar almost every headland
and low point of sand has a coral−reef extending from it in a S.W. and N.E. line, parallel to the currents on
that shore. I should think the influence of the currents chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of the coral. Round many intertropical islands, for
instance the Abrolhos on the coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by Mr.
Cuming, round the Philippines, the bottom of the sea is entirely coated by irregular masses of coral, which
although often of large size, do not reach the surface and form proper reefs. This must be owing, either to
insufficient growth, or to the absence of those kinds of corals which can withstand the breaking of the waves.
The three classes, atoll−formed, barrier, and fringing−reefs, together with the modifications just described of
the latter, include all the most remarkable coral formations anywhere existing. At the commencement of the
last chapter in the volume, where I detail the principles on which the map (Plate III.) is coloured, the
exceptional cases will be enumerated.

Chapter IV

On the distribution and growth of coral - reefs

In this chapter I will give all the facts which I have collected, relating to the distribution of coral−reefs,−−to
the conditions favourable to their increase,−−to the rate of their growth,−−and to the depth at which they are
formed.
These subjects have an important bearing on the theory of the origin of the different classes of coral−reefs.

SECTION 4.I.−−ON THE DISTRIBUTION OF CORAL−REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE.

With regard to the limits of latitude, over which coral−reefs extend, I have nothing new to add. The Bermuda
Islands, in 32 deg 15' N., is the point furthest removed from the equator, in which they appear to exist; and it
has been suggested that their extension so far northward in this instance is owing to the warmth of the Gulf
Stream. In the Pacific, the Loo Choo Islands, in latitude 27 deg N., have reefs on their shores, and there is an
atoll in 28 deg 30', situated N.W. of the Sandwich Archipelago. In the Red Sea there are coral−reefs in latitude
30 deg. In the southern hemisphere coral−reefs do not extend so far from the equatorial sea. In the Southern
Pacific there are only a few reefs beyond the line of the tropics, but Houtmans Abrolhos, on the western
shores of Australia in latitude 29 deg S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved from it, has been thought to be
favourable to the increase of coral−reefs. There is, however, not much foundation for this view; for nowhere
are coral−reefs more extensive than on the shores of New Caledonia, and of north−eastern Australia, which
consist of primary formations; and in the largest groups of atolls, namely the Maldiva, Chagos, Marshall,
Gilbert, and Low Archipelagoes, there is no volcanic or other kind of rock, excepting that formed of coral.
The entire absence of coral−reefs in certain large areas within the tropical seas, is a remarkable fact. Thus no
coral−reefs were observed, during the surveying voyages of the "Beagle" and her tender on the west coast of
South America south of the equator, or round the Galapagos Islands. It appears, also, that there are none (I
have been informed that this is the case, by Lieutenant Ryder, R.N., and others who have had ample
opportunities for observation.) north of the equator; Mr. Lloyd, who surveyed the Isthmus of Panama,
remarked to me, that although he had seen corals living in the Bay of Panama, yet he had never observed any
reefs formed by them. I at first attributed this absence of reefs on the coasts of Peru and of the Galapagos
Islands (The mean temperature of the surface sea from observations made by the direction of Captain Fitzroy
on the shores of the Galapagos Islands, between the 16th of September and the 20th of October, 1835, was 68
deg Fahr. The lowest temperature observed was 58.5 deg at the south−west end of Albemarle Island; and on
the west coast of this island, it was several times 62 deg and 63 deg. The mean temperature of the sea in the
Low Archipelago of atolls, and near Tahiti, from similar observations made on board the "Beagle", was
(although further from the equator) 77.5 deg, the lowest any day being 76.5 deg. Therefore we have here a
difference of 9.5 deg in mean temperature, and 18 deg in extremes; a difference doubtless quite sufficient to
affect the distribution of organic beings in the two areas.), to the coldness of the currents from the south, but
the Gulf of Panama is one of the hottest pelagic districts in the world. (Humboldt's "Personal Narrative,"
volume vii., page 434.) In the central parts of the Pacific there are islands entirely free from reefs; in some few
of these cases I have thought that this was owing to recent volcanic action; but the existence of reefs round the
greater part of Hawaii, one of the Sandwich Islands, shows that recent volcanic action does not necessarily
prevent their growth.
In the last chapter I stated that the bottom of the sea round some islands is thickly coated with living corals,
which nevertheless do not form reefs, either from insufficient growth, or from the species not being adapted to
contend with the breaking waves.
I have been assured by several people, that there are no coral−reefs on the west coast of Africa (It might be
concluded, from a paper by Captain Owen ("Geographical Journal", volume ii., page 89), that the reefs off
Cape St. Anne and the Sherboro' Islands were of coral, although the author states that they are not purely
coralline. But I have been assured by Lieutenant Holland, R.N., that these reefs are not of coral, or at least that
they do not at all resemble those in the West Indies.), or round the islands in the Gulf of Guinea. This perhaps
may be attributed, in part, to the sediment brought down by the many rivers debouching on that coast, and to
the extensive mud−banks, which line great part of it. But the islands of St. Helena, Ascension, the Cape
Verdes, St. Paul's, and Fernando Noronha, are, also, entirely without reefs, although they lie far out at sea, are
composed of the same ancient volcanic rocks, and have the same general form, with those islands in the
Pacific, the shores of which are surrounded by gigantic walls of coral−rock. With the exception of Bermuda,
there is not a single coral−reef in the central expanse of the Atlantic Ocean. It will, perhaps, be suggested that
the quantity of carbonate of lime in different parts of the sea, may regulate the presence of reefs. But this
cannot be the case, for at Ascension, the waves charged to excess precipitate a thick layer of calcareous matter
on the tidal rocks; and at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant on the shores,
but it forms the chief part of some upraised post−tertiary strata. The apparently capricious distribution,
therefore, of coral−reefs, cannot be explained by any of these obvious causes; but as the study of the terrestrial
and better known half of the world must convince every one that no station capable of supporting life is
lost,−−nay more, that there is a struggle for each station, between the different orders of nature,−−we may
conclude that in those parts of the intertropical sea, in which there are no coral−reefs, there are other organic
bodies supplying the place of the reef−building polypifers. It has been shown in the chapter on Keeling atoll
that there are some species of large fish, and the whole tribe of Holothuriae which prey on the tenderer parts
of the corals. On the other hand, the polypifers in their turn must prey on some other organic beings; the
decrease of which from any cause would cause a proportionate destruction of the living coral. The relations,
therefore, which determine the formation of reefs on any shore, by the vigorous growth of the efficient kinds
of coral, must be very complex, and with our imperfect knowledge quite inexplicable. From these
considerations, we may infer that changes in the condition of the sea, not obvious to our senses, might destroy
all the coral−reefs in one area, and cause them to appear in another: thus, the Pacific or Indian Ocean might
become as barren of coral−reefs as the Atlantic now is, without our being able to assign any adequate cause
for such a change.
It has been a question with some naturalists, which part of a reef is most favourable to the growth of coral.
The great mounds of living Porites and of Millepora round Keeling atoll occur exclusively on the extreme
verge of the reef, which is washed by a constant succession of breakers; and living coral nowhere else forms
solid masses. At the Marshall islands the larger kinds of coral (chiefly species of Astraea, a genus closely
allied to Porites) "which form rocks measuring several fathoms in thickness," prefer, according to Chamisso
(Kotzebue's "First Voyage" (English Translation), volume iii., pages 142, 143, 331.), the most violent surf. I
have stated that the outer margin of the Maldiva atolls consists of living corals (some of which, if not all, are
of the same species with those at Keeling atoll), and here the surf is so tremendous, that even large ships have
been thrown, by a single heave of the sea, high and dry on the reef, all on board thus escaping with their lives.
Ehrenberg (Ehrenberg, "Uber die Natur und Bildung der Corallen Banke im rothen Meere," page 49.)
remarks, that in the Red Sea the strongest corals live on the outer reefs, and appear to love the surf; he adds,
that the more branched kinds abound a little way within, but that even these in still more protected places,
become smaller. Many other facts having a similar tendency might be adduced. (In the West Indies, as I am
informed by Captain Bird Allen, R.N., it is the common belief of those, who are best acquainted with the
reefs, that the coral flourishes most, where freely exposed to the swell of the open sea.) It has, however, been
doubted by MM. Quoy and Gaimard, whether any kind of coral can even withstand, much less flourish in, the
breakers of an open sea ("Annales des Sciences Naturelles," tome vi., pages 276, 278.−−"La ou les ondes sont
agitees, les Lytophytes ne peuvent travailler, parce qu'elles detruiraient leurs fragiles edifices," etc.): they
affirm that the saxigenous lithophytes flourish only where the water is tranquil, and the heat intense. This
statement has passed from one geological work to another; nevertheless, the protection of the whole reef
undoubtedly is due to those kinds of coral, which cannot exist in the situations thought by these naturalists to
be most favourable to them. For should the outer and living margin perish, of any one of the many low
coral−islands, round which a line of great breakers is incessantly foaming, the whole, it is scarcely possible to
doubt, would be washed away and destroyed, in less than half a century. But the vital energies of the corals
conquer the mechanical power of the waves; and the large fragments of reef torn up by every storm, are
replaced by the slow but steady growth of the innumerable polypifers, which form the living zone on its outer
edge.
From these facts, it is certain, that the strongest and most massive corals flourish, where most exposed. The
less perfect state of the reef of most atolls on the leeward and less exposed side, compared with its state to
windward; and the analogous case of the greater number of breaches on the near sides of those atolls in the
Maldiva Archipelago, which afford some protection to each other, are obviously explained by this
circumstance. If the question had been, under what conditions the greater number of species of coral, not
regarding their bulk and strength, were developed, I should answer,−−probably in the situations described by
MM. Quoy and Gaimard, where the water is tranquil and the heat intense. The total number of species of coral
in the circumtropical seas must be very great: in the Red Sea alone, 120 kinds, according to Ehrenberg
(Ehrenberg, "Uber die Natur," etc., etc., page 46.), have been observed.
The same author has observed that the recoil of the sea from a steep shore is injurious to the growth of coral,
although waves breaking over a bank are not so. Ehrenberg also states, that where there is much sediment,
placed so as to be liable to be moved by the waves there is little or no coral; and a collection of living
specimens placed by him on a sandy shore died in the course of a few days. (Ibid., page 49.) An experiment,
however, will presently be related in which some large masses of living coral increased rapidly in size, after
having been secured by stakes on a sandbank. That loose sediment should be injurious to the living polypifers,
appears, at first sight, probable; and accordingly, in sounding off Keeling atoll, and (as will hereafter be
shown) off Mauritius, the arming of the lead invariably came up clean, where the coral was growing
vigorously. This same circumstance has probably given rise to a strange belief, which, according to Captain
Owen (Captain Owen on the Geography of the Maldiva Islands, "Geographical Journal", volume ii., page
88.), is general amongst the inhabitants of the Maldiva atolls, namely that corals have roots, and therefore that
if merely broken down to the surface, they grow up again; but, if rooted out, they are permanently destroyed.
By this means the inhabitants keep their harbours clear; and thus the French Governor of St. Mary's in
Madagascar, "cleared out and made a beautiful little port at that place." For it is probable that sand would
accumulate in the hollows formed by tearing out the corals, but not on the broken and projecting stumps, and
therefore, in the former case, the fresh growth of the coral might be thus prevented.
In the last chapter I remarked that fringing−reefs are almost universally breached, where streams enter the sea.
(Lieutenant Wellstead and others have remarked that this is the case in the Red Sea; Dr. Ruppell ("Reise in
Abyss." Band. i., page 142) says that there are pear−shaped harbours in the upraised coral−coast, into which
periodical streams enter. From this circumstance, I presume, we must infer that before the upheaval of the
strata now forming the coast−land, fresh water and sediment entered the sea at these points; and the coral
being thus prevented growing, the pear−shaped harbours were produced.) Most authors have attributed this
fact to the injurious effects of the fresh water, even where it enters the sea only in small quantity, and during a
part of the year. No doubt brackish water would prevent or retard the growth of coral; but I believe that the
mud and sand which is deposited, even by rivulets when flooded, is a much more efficient check. The reef on
each side of the channel leading into Port Louis at Mauritius, ends abruptly in a wall, at the foot of which I
sounded and found a bed of thick mud. This steepness of the sides appears to be a general character in such
breaches. Cook (Cook's "First Voyage," volume ii., page 271 (Hawkesworth's edition).), speaking of one at
Raiatea, says, "like all the rest, it is very steep on both sides." Now, if it were the fresh water mingling with
the salt which prevented the growth of coral, the reef certainly would not terminate abruptly, but as the
polypifers nearest the impure stream would grow less vigorously than those farther off, so would the reef
gradually thin away. On the other hand, the sediment brought down from the land would only prevent the
growth of the coral in the line of its deposition, but would not check it on the side, so that the reefs might
increase till they overhung the bed of the channel. The breaches are much fewer in number, and front only the
larger valleys in reefs of the encircling barrier class. They probably are kept open in the same manner as those
into the lagoon of an atoll, namely, by the force of the currents and the drifting outwards of fine sediment.
Their position in front of valleys, although often separated from the land by deep water lagoon−channels,
which it might be thought would entirely remove the injurious effects both of the fresh water and the
sediment, will receive a simple explanation when we discuss the origin of barrier−reefs.
In the vegetable kingdom every different station has its peculiar group of plants, and similar relations appear
to prevail with corals. We have already described the great difference between the corals within the lagoon of
an atoll and those on its outer margin. The corals, also, on the margin of Keeling Island occurred in zones;
thus the Porites and Millepora complanata grow to a large size only where they are washed by a heavy sea,
and are killed by a short exposure to the air; whereas, three species of Nullipora also live amidst the breakers,
but are able to survive uncovered for a part of each tide; at greater depths, a strong Madrepora and Millepora
alcicornis are the commonest kinds, the former appearing to be confined to this part, beneath the zone of
massive corals, minute encrusting corallines and other organic bodies live. If we compare the external margin
of the reef at Keeling atoll with that on the leeward side of Mauritius, which are very differently
circumstanced, we shall find a corresponding difference in the appearance of the corals. At the latter place, the
genus Madrepora is preponderant over every other kind, and beneath the zone of massive corals there are
large beds of Seriatopora. There is also a marked difference, according to Captain Moresby (Captain Moresby
on the Northern Maldiva atolls, "Geographical Journal", volume v., page 401.), between the great branching
corals of the Red Sea, and those on the reefs of the Maldiva atolls.
These facts, which in themselves are deserving of notice, bear, perhaps, not very remotely, on a remarkable
circumstance which has been pointed out to me by Captain Moresby, namely, that with very few exceptions,
none of the coral−knolls within the lagoons of Peros Banhos, Diego Garcia, and the Great Chagos Bank (all
situated in the Chagos group), rise to the surface of the water; whereas all those, with equally few exceptions,
within Solomon and Egmont atolls in the same group, and likewise within the large southern Maldiva atolls,
reach the surface. I make these statements, after having examined the charts of each atoll. In the lagoon of
Peros Banhos, which is nearly twenty miles across, there is only one single reef which rises to the surface; in
Diego Garcia there are seven, but several of these lie close to the margin of the lagoon, and need scarcely have
been reckoned; in the Great Chagos Bank there is not one. On the other hand, in the lagoons of some of the
great southern Maldiva atolls, although thickly studded with reefs, every one without exception rises to the
surface; and on an average there are less than two submerged reefs in each atoll; in the northern atolls,
however, the submerged lagoon−reefs are not quite so rare. The submerged reefs in the Chagos atolls
generally have from one to seven fathoms water on them, but some have from seven to ten. Most of them are
small with very steep sides (Some of these statements were not communicated to me verbally by Captain
Moresby, but are taken from the MS. account before alluded to, of the Chagos Group.); at Peros Banhos they
rise from a depth of about thirty fathoms, and some of them in the Great Chagos Bank from above forty
fathoms; they are covered, Captain Moresby informs me, with living and healthy coral, two and three feet
high, consisting of several species. Why then have not these lagoon−reefs reached the surface, like the
innumerable ones in the atolls above named? If we attempt to assign any difference in their external
conditions, as the cause of this diversity, we are at once baffled. The lagoon of Diego Garcia is not deep, and
is almost wholly surrounded by its reef; Peros Banhos is very deep, much larger, with many wide passages
communicating with the open sea. On the other hand, of those atolls, in which all or nearly all the
lagoon−reefs have reached the surface, some are small, others large, some shallow, others deep, some
well−enclosed, and others open.
Captain Moresby informs me that he has seen a French chart of Diego Garcia made eighty years before his
survey, and apparently very accurate; and from it he infers, that during this interval there has not been the
smallest change in the depth on any of the knolls within the lagoon. It is also known that during the last
fifty−one years, the eastern channel into the lagoon has neither become narrower, nor decreased in depth; and
as there are numerous small knolls of living coral within it, some change might have been anticipated.
Moreover, as the whole reef round the lagoon of this atoll has been converted into land−−an unparalleled case,
I believe, in an atoll of such large size,−−and as the strip of land is for considerable spaces more than half a
mile wide−−also a very unusual circumstance,−−we have the best possible evidence, that Diego Garcia has
remained at its present level for a very long period. With this fact, and with the knowledge that no sensible
change has taken place during eighty years in the coral−knolls, and considering that every single reef has
reached the surface in other atolls, which do not present the smallest appearance of being older than Diego
Garcia and Peros Banhos, and which are placed under the same external conditions with them, one is led to
conclude that these submerged reefs, although covered with luxuriant coral, have no tendency to grow
upwards, and that they would remain at their present levels for an almost indefinite period.
From the number of these knolls, from their position, size, and form, many of them being only one or two
hundred yards across, with a rounded outline, and precipitous sides,−−it is indisputable that they have been
formed by the growth of coral; and this makes the case much more remarkable. In Peros Banhos and in the
Great Chagos Bank, some of these almost columnar masses are 200 feet high, and their summits lie only from
two to eight fathoms beneath the surface; therefore, a small proportional amount more of growth would cause
them to attain the surface, like those numerous knolls, which rise from an equally great depth within the
Maldiva atolls. We can hardly suppose that time has been wanting for the upward growth of the coral, whilst
in Diego Garcia, the broad annular strip of land, formed by the continued accumulation of detritus, shows how
long this atoll has remained at its present level. We must look to some other cause than the rate of growth; and
I suspect it will be found in the reefs being formed of different species of corals, adapted to live at different
depths.
The Great Chagos Bank is situated in the centre of the Chagos Group, and the Pitt and Speaker Banks at its
two extreme points. These banks resemble atolls, except in their external rim being about eight fathoms
submerged, and in being formed of dead rock, with very little living coral on it: a portion nine miles long of
the annular reef of Peros Banhos atoll is in the same condition. These facts, as will hereafter be shown, render
it very probable that the whole group at some former period subsided seven or eight fathoms; and that the
coral perished on the outer margin of those atolls which are now submerged, but that it continued alive, and
grew up to the surface on those which are now perfect. If these atolls did subside, and if from the suddenness
of the movement or from any other cause, those corals which are better adapted to live at a certain depth than
at the surface, once got possession of the knolls, supplanting the former occupants, they would exert little or
no tendency to grow upwards. To illustrate this, I may observe, that if the corals of the upper zone on the outer
edge of Keeling atoll were to perish, it is improbable that those of the lower zone would grow to the surface,
and thus become exposed to conditions for which they do not appear to be adapted. The conjecture, that the
corals on the submerged knolls within the Chagos atolls have analogous habits with those of the lower zone
outside Keeling atoll, receives some support from a remark by Captain Moresby, namely, that they have a
different appearance from those on the reefs in the Maldiva atolls, which, as we have seen, all rise to the
surface: he compares the kind of difference to that of the vegetation under different climates. I have entered at
considerable length into this case, although unable to throw much light on it, in order to show that an equal
tendency to upward growth ought not to be attributed to all coral−reefs,−−to those situated at different
depths,−−to those forming the ring of an atoll or those on the knolls within a lagoon,−−to those in one area
and those in another. The inference, therefore, that one reef could not grow up to the surface within a given
time, because another, not known to be covered with the same species of corals, and not known to be placed
under conditions exactly the same, has not within the same time reached the surface, is unsound.
SECTION 4.II.−−ON THE RATE OF GROWTH OF CORAL−REEFS.
The remark made at the close of the last section, naturally leads to this division of our subject, which has not, I
think, hitherto been considered under a right point of view. Ehrenberg (Ehrenberg, as before cited, pages 39,
46, and 50.) has stated, that in the Red Sea, the corals only coat other rocks in a layer from one to two feet in
thickness, or at most to a fathom and a half; and he disbelieves that, in any case, they form, by their own
proper growth, great masses, stratum over stratum. A nearly similar observation has been made by MM. Quoy
and Gaimard ("Annales des Sciences Nat." tom. vi., page 28.), with respect to the thickness of some upraised
beds of coral, which they examined at Timor and some other places. Ehrenberg (Ehrenberg, ut sup., page 42.)
saw certain large massive corals in the Red Sea, which he imagines to be of such vast antiquity, that they
might have been beheld by Pharaoh; and according to Mr. Lyell (Lyell's "Principles of Geology," book iii.,
chapter xviii.) there are certain corals at Bermuda, which are known by tradition, to have been living for
centuries. To show how slowly coral−reefs grow upwards, Captain Beechey (Beechey's "Voyage to the
Pacific," chapter viii.) has adduced the case of the Dolphin Reef off Tahiti, which has remained at the same
depth beneath the surface, namely about two fathoms and a half, for a period of sixty−seven years. There are
reefs in the Red Sea, which certainly do not appear (Ehrenberg, ut sup., page 43.) to have increased in
dimensions during the last half−century, and from the comparison of old charts with recent surveys, probably
not during the last two hundred years. These, and other similar facts, have so strongly impressed many with
the belief of the extreme slowness of the growth of corals, that they have even doubted the possibility of
islands in the great oceans having been formed by their agency.

overwhelmed by this difficulty, have admitted that it would require thousands, and tens of thousands of years,
to form a mass, even of inconsiderable thickness; but the subject has not, I believe, been viewed in the proper
light.
That masses of considerable thickness have been formed by the growth of coral, may be inferred with
certainty from the following facts. In the deep lagoons of Peros Banhos and of the Great Chagos Bank, there
are, as already described, small steep−sided knolls covered with living coral. There are similar knolls in the
southern Maldiva atolls, some of which, as Captain Moresby assures me, are less than a hundred yards in
diameter, and rise to the surface from a depth of between two hundred and fifty and three hundred feet.
Considering their number, form, and position, it would be preposterous to suppose that they are based on
pinnacles of any rock, not of coral formation; or that sediment could have been heaped up into such small and
steep isolated cones. As no kind of living coral grows above the height of a few feet, we are compelled to
suppose that these knolls have been formed by the successive growth and death of many individuals,−−first
one being broken off or killed by some accident, and then another, and one set of species being replaced by
another set with different habits, as the reef rose nearer the surface, or as other changes supervened. The
spaces between the corals would become filled up with fragments and sand, and such matter would probably
soon be consolidated, for we learn from Lieutenant Nelson ("Geological Transactions," volume v., page 113.),
that at Bermuda a process of this kind takes place beneath water, without the aid of evaporation. In reefs, also,
of the barrier class, we may feel sure, as I have shown, that masses of great thickness have been formed by the
growth of the coral; in the case of Vanikoro, judging only from the depth of the moat between the land and the
reef, the wall of coral−rock must be at least three hundred feet in vertical thickness.
It is unfortunate that the upraised coral−islands in the Pacific have not been examined by a geologist. The
cliffs of Elizabeth Island, in the Low Archipelago, are eighty feet high, and appear, from Captain Beechey's
description, to consist of a homogeneous coral−rock. From the isolated position of this island, we may safely
infer that it is an upraised atoll, and therefore that it has been formed by masses of coral, grown together.
Savage Island seems, from the description of the younger Forster (Forster's "Voyage round the World with
Cook," volume ii., pages 163, 167.), to have a similar structure, and its shores are about forty feet high: some
of the Cook Islands also appear (Williams's "Narrative of Missionary Enterprise," page 30.) to be similarly
composed. Captain Belcher, R.N., in a letter which Captain Beaufort showed me at the admiralty, speaking of
Bow atoll, says, "I have succeeded in boring forty−five feet through coral−sand, when the auger became
jammed by the falling in of the surrounding CREAMY matter." On one of the Maldiva atolls, Captain
Moresby bored to a depth of twenty−six feet, when his auger also broke: he has had the kindness to give me
the matter brought up; it is perfectly white, and like finely triturated coral−rock.
In my description of Keeling atoll, I have given some facts, which show that the reef probably has grown
outwards; and I have found, just within the outer margin, the great mounds of Porites and of Millepora, with
their summits lately killed, and their sides subsequently thickened by the growth of the coral: a layer, also, of
Nullipora had already coated the dead surface. As the external slope of the reef is the same round the whole of
this atoll, and round many other atolls, the angle of inclination must result from an adaption between the
growing powers of the coral, and the force of the breakers, and their action on the loose sediment. The reef,
therefore, could not increase outwards, without a nearly equal addition to every part of the slope, so that the
original inclination might be preserved, and this would require a large amount of sediment, all derived from
the wear of corals and shells, to be added to the lower part. Moreover, at Keeling atoll, and probably in many
other cases, the different kinds of corals would have to encroach on each other; thus the Nulliporae cannot
increase outwards without encroaching on the Porites and Millepora complanata, as is now taking place; nor
these latter without encroaching on the strongly branched Madreporet, the Millepora alcicornis, and some
Astraeas; nor these again without a foundation being formed for them within the requisite depth, by the
accumulation of sediment. How slow, then, must be the ordinary lateral or outward growth of such reefs. But
off Christmas atoll, where the sea is much more shallow than is usual, we have good reason to believe that,
within a period not very remote, the reef has increased considerably in width. The land has the extraordinary
breadth of three miles; it consists of parallel ridges of shells and broken corals, which furnish "an
incontestable proof," as observed by Cook (Cook's "Third Voyage," book III., chapter x.), "that the island has
been produced by accessions from the sea, and is in a state of increase." The land is fronted by a coral−reef,
and from the manner in which islets are known to be formed, we may feel confident that the reef was not three
miles wide, when the first, or most backward ridge, was thrown up; and, therefore, we must conclude that the
reef has grown outwards during the accumulation of the successive ridges. Here then, a wall of coral−rock of
very considerable breadth has been formed by the outward growth of the living margin, within a period during
which ridges of shells and corals, lying on the bare surface, have not decayed. There can be little doubt, from
the account given by Captain Beechey, that Matilda atoll, in the Low Archipelago, has been converted in the
space of thirty−four years, from being, as described by the crew of a wrecked whaling vessel, a "reef of rocks"
into a lagoon−island, fourteen miles in length, with "one of its sides covered nearly the whole way with high
trees." (Beechey's "Voyage to the Pacific," chapter vii. and viii.) The islets, also, on Keeling atoll, it has been
shown, have increased in length, and since the construction of an old chart, several of them have become
united into one long islet; but in this case, and in that of Matilda atoll, we have no proof, and can only infer as
probable, that the reef, that is the foundation of the islets, has increased as well as the islets themselves.
After these considerations, I attach little importance, as indicating the ordinary and still less the possible rate
of OUTWARD growth of coral−reefs, to the fact that certain reefs in the Red Sea have not increased during a
long interval of time; or to other such cases, as that of Ouluthy atoll in the Caroline group, where every islet,
described a thousand years before by Cantova was found in the same state by Lutke (F. Lutke's "Voyage
autour du Monde." In the group Elato, however, it appears that what is now the islet Falipi, is called in
Cantova's Chart, the Banc de Falipi. It is not stated whether this has been caused by the growth of coral, or by
the accumulation of sand.),−−without it could be shown that, in these cases, the conditions were favourable to
the vigorous and unopposed growth of the corals living in the different zones of depth, and that a proper basis
for the extent of the reef was present. The former conditions must depend on many contingencies, and in the
deep oceans where coral formations most abound, a basis within the requisite depth can rarely be present.
Nor do I attach any importance to the fact of certain submerged reefs, as those off Tahiti, or those within
Diego Garcia not now being nearer the surface than they were many years ago, as an indication of the rate
under favourable circumstances of the UPWARD growth of reefs; after it has been shown, that all the reefs
have grown to the surface in some of the Chagos atolls, but that in neighbouring atolls which appear to be of
equal antiquity and to be exposed to the same external conditions, every reef remains submerged; for we are
almost driven to attribute this to a difference, not in the rate of growth, but in the habits of the corals in the
two cases.
In an old−standing reef, the corals, which are so different in kind on different parts of it, are probably all
adapted to the stations they occupy, and hold their places, like other organic beings, by a struggle one with
another, and with external nature; hence we may infer that their growth would generally be slow, except under
peculiarly favourable circumstances. Almost the only natural condition, allowing a quick upward growth of
the whole surface of a reef, would be a slow subsidence of the area in which it stood; if, for instance, Keeling
atoll were to subside two or three feet, can we doubt that the projecting margin of live coral, about half an
inch in thickness, which surrounds the dead upper surfaces of the mounds of Porites, would in this case form a
concentric layer over them, and the reef thus increase upwards, instead of, as at present, outwards? The
Nulliporae are now encroaching on the Porites and Millepora, but in this case might we not confidently expect
that the latter would, in their turn, encroach on the Nulliporae? After a subsidence of this kind, the sea would
gain on the islets, and the great fields of dead but upright corals in the lagoon, would be covered by a sheet of
clear water; and might we not then expect that these reefs would rise to the surface, as they anciently did when
the lagoon was less confined by islets, and as they did within a period of ten years in the schooner−channel,
cut by the inhabitants? In one of the Maldiva atolls, a reef, which within a very few years existed as an islet
bearing cocoa−nut trees, was found by Lieutenant Prentice "ENTIRELY COVERED WITH LIVE CORAL
AND MADREPORE." The natives believe that the islet was washed away by a change in the currents, but if,
instead of this, it had quietly subsided, surely every part of the island which offered a solid foundation, would
in a like manner have become coated with living coral.
Through steps such as these, any thickness of rock, composed of a singular intermixture of various kinds of
corals, shells, and calcareous sediment, might be formed; but without subsidence, the thickness would
necessarily be determined by the depth at which the reef−building polypifers can exist. If it be asked, at what
rate in years I suppose a reef of coral favourably circumstanced could grow up from a given depth; I should
answer, that we have no precise evidence on this point, and comparatively little concern with it. We see, in
innumerable points over wide areas, that the rate has been sufficient, either to bring up the reefs from various
depths to the surface, or, as is more probable, to keep them at the surface, during progressive subsidences; and
this is a much more important standard of comparison than any cycle of years.
It may, however, be inferred from the following facts, that the rate in years under favourable circumstances
would be very far from slow. Dr. Allan, of Forres, has, in his MS. Thesis deposited in the library of the
Edinburgh University (extracts from which I owe to the kindness of Dr. Malcolmson), the following account
of some experiments, which he tried during his travels in the years 1830 to 1832 on the east coast of
Madagascar. "To ascertain the rise and progress of the coral−family, and fix the number of species met with at
Foul Point (latitude 17 deg 40') twenty species of coral were taken off the reef and planted apart on a
sand−bank THREE FEET DEEP AT LOW WATER. Each portion weighed ten pounds, and was kept in its
place by stakes. Similar quantities were placed in a clump and secured as the rest. This was done in December
1830. In July following, each detached mass was nearly level with the sea at low water, quite immovable, and
several feet long, stretching as the parent reef, with the coast current from north to south. The masses
accumulated in a clump were found equally increased, but some of the species in such unequal ratios, as to be
growing over each other." The loss of Dr. Allan's magnificent collection by shipwreck, unfortunately prevents
its being known to what genera these corals belonged; but from the numbers experimented on, it is certain that
all the more conspicuous kinds must have been included. Dr. Allan informs me, in a letter, that he believes it
was a Madrepora, which grew most vigorously. One may be permitted to suspect that the level of the sea
might possibly have been somewhat different at the two stated periods; nevertheless, it is quite evident that
the growth of the ten−pound masses, during the six or seven months, at the end of which they were found
immovably fixed (It is stated by De la Beche ("Geological Manual," page 143), on the authority of Mr. Lloyd,
who surveyed the Isthmus of Panama, that some specimens of Polypifers, placed by him in a sheltered pool of
water, were found in the course of a few days firmly fixed by the secretion of a stony matter, to the bottom)
and several feet in length, must have been very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very interesting, as it shows the manner in which
a reef, supporting many species of coral, would probably be affected by a change in the external conditions
favouring one kind more than another. The growth of the masses of coral in N. and S. lines parallel to the
prevailing currents, whether due to the drifting of sediment or to the simple movement of the water, is, also, a
very interesting circumstance.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some degree corroborates the result of Dr.
Allan's experiments: it is, that in the Persian Gulf a ship had her copper bottom encrusted in the course of
twenty months with a layer of coral, TWO FEET in thickness, which it required great force to remove, when
the vessel was docked: it was not ascertained to what order this coral belonged. The case of the
schooner−channel choked up with coral in an interval of less than ten years, in the lagoon of Keeling atoll,
should be here borne in mind. We may also infer, from the trouble which the inhabitants of the Maldiva atolls
take to root out, as they express it, the coral−knolls from their harbours, that their growth can hardly be very
slow. (Mr. Stutchbury ("West of England Journal", No. I., page 50.) has described a specimen of Agaricia,
"weighing 2 lbs. 9 oz., which surrounds a species of oyster, whose age could not be more than two years, and
yet is completely enveloped by this dense coral." I presume that the oyster was living when the specimen was
procured; otherwise the fact tells nothing. Mr. Stutchbury also mentions an anchor, which had become entirely
encrusted with coral in fifty years; other cases, however, are recorded of anchors which have long remained
amidst coral−reefs without having become coated. The anchor of the "Beagle", in 1832, after having been
down exactly one month at Rio de Janeiro, was so thickly coated by two species of Tubularia, that large
spaces of the iron were entirely concealed; the tufts of this horny zoophyte were between two and three inches
in length. It has been attempted to compute, but I believe erroneously, the rate of growth of a reef, from the
fact mentioned by Captain Beechey, of the Chama gigas being embedded in coral−rock. But it should be
remembered, that some species of this genus invariably live, both whilst young and old, in cavities, which the
animal has the power of enlarging with its growth. I saw many of these shells thus embedded in the outer
"flat" of Keeling atoll, which is composed of dead rock; and therefore the cavities in this case had no relation
whatever with the growth of coral. M. Lesson, also, speaking of this shell (Partie Zoolog. "Voyage de la
'Coquille'"), has remarked, "que constamment ses valves etaient engages completement dans la masse des
Madrepores.")
From the facts given in this section, it may be concluded, first, that considerable thicknesses of rock have
certainly been formed within the present geological area by the growth of coral and the accumulation of its
detritus; and, secondly, that the increase of individual corals and of reefs, both outwards or horizontally and
upwards or vertically, under the peculiar conditions favourable to such increase, is not slow, when referred
either to the standard of the average oscillations of level in the earth's crust, or to the more precise but less
important one of a cycle of years.
SECTION 4.III.−−ON THE DEPTHS AT WHICH REEF−BUILDING POLYPIFERS CAN LIVE.
I have already described in detail, which might have appeared trivial, the nature of the bottom of the sea
immediately surrounding Keeling atoll; and I will now describe with almost equal care the soundings off the
fringing−reefs of Mauritius. I have preferred this arrangement, for the sake of grouping together facts of a
similar nature. I sounded with the wide bell−shaped lead which Captain Fitzroy used at Keeling Island, but
my examination of the bottom was confined to a few miles of coast (between Port Louis and Tomb Bay) on
the leeward side of the island. The edge of the reef is formed of great shapeless masses of branching
Madrepores, which chiefly consist of two species,−−apparently M. corymbosa and pocillifera,−− mingled
with a few other kinds of coral. These masses are separated from each other by the most irregular gullies and
cavities, into which the lead sinks many feet. Outside this irregular border of Madrepores, the water deepens
gradually to twenty fathoms, which depth generally is found at the distance of from half to three−quarters of a
mile from the reef. A little further out the depth is thirty fathoms, and thence the bank slopes rapidly into the
depths of the ocean. This inclination is very gentle compared with that outside Keeling and other atolls, but
compared with most coasts it is steep. The water was so clear outside the reef, that I could distinguish every
object forming the rugged bottom. In this part, and to a depth of eight fathoms, I sounded repeatedly, and at
each cast pounded the bottom with the broad lead, nevertheless the arming invariably came up perfectly clean,
but deeply indented. From eight to fifteen fathoms a little calcareous sand was occasionally brought up, but
more frequently the arming was simply indented. In all this space the two Madrepores above mentioned, and
two species of Astraea, with rather large stars, seemed the commonest kinds (Since the preceding pages were
printed off, I have received from Mr. Lyell a very interesting pamphlet, entitled "Remarks upon Coral
Formations," etc., by J. Couthouy, Boston, United States, 1842. There is a statement (page 6), on the authority
of the Rev. J. Williams, corroborating the remarks made by Ehrenberg and Lyell (page 71 of this volume), on
the antiquity of certain individual corals in the Red Sea and at Bermuda; namely, that at Upolu, one of the
Navigator Islands, "particular clumps of coral are known to the fishermen by name, derived from either some
particular configuration or tradition attached to them, and handed down from time immemorial." With respect
to the thickness of masses of coral−rock, it clearly appears, from the descriptions given by Mr. Couthouy
(pages 34, 58) that Mangaia and Aurora Islands are upraised atolls, composed of coral rock: the level summit
of the former is about three hundred feet, and that of Aurora Island is two hundred feet above the sea−level.);
and it must be noticed that twice at the depth of fifteen fathoms, the arming was marked with a clean
impression of an Astraea. Besides these lithophytes, some fragments of the Millepora alcicornis, which occurs
in the same relative position at Keeling Island, were brought up; and in the deeper parts there were large beds
of a Seriatopora, different from S. subulata, but closely allied to it. On the beach within the reef, the rolled
fragments consisted chiefly of the corals just mentioned, and of a massive Porites, like that at Keeling atoll, of
a Meandrina, Pocillopora verrucosa, and of numerous fragments of Nullipora. From fifteen to twenty fathoms
the bottom was, with few exceptions, either formed of sand, or thickly covered with Seriatopora: this delicate
coral seems to form at these depths extensive beds unmingled with any other kind. At twenty fathoms, one
sounding brought up a fragment of Madrepora apparently M. pocillifera, and I believe it is the same species
(for I neglected to bring specimens from both stations) which mainly forms the upper margin of the reef; if so,
it grows in depths varying from 0 to 20 fathoms. Between 20 and 23 fathoms I obtained several soundings,
and they all showed a sandy bottom, with one exception at 30 fathoms, when the arming came up scooped out,
as if by the margin of a large Caryophyllia. Beyond 33 fathoms I sounded only once; and from 86 fathoms, at
the distance of one mile and a third from the edge of the reef, the arming brought up calcareous sand with a
pebble of volcanic rock. The circumstance of the arming having invariably come up quite clean, when
sounding within a certain number of fathoms off the reefs of Mauritius and Keeling atoll (eight fathoms in the
former case, and twelve in the latter) and of its having always come up (with one exception) smoothed and
covered with sand, when the depth exceeded twenty fathoms, probably indicates a criterion, by which the
limits of the vigorous growth of coral might in all cases be readily ascertained. I do not, however, suppose that
if a vast number of soundings were obtained round these islands, the limit above assigned would be found
never to vary, but I conceive the facts are sufficient to show, that the exceptions would be few. The
circumstance of a GRADUAL change, in the two cases, from a field of clean coral to a smooth sandy bottom,
is far more important in indicating the depth at which the larger kinds of coral flourish than almost any
number of separate observations on the depth, at which certain species have been dredged up. For we can
understand the gradation, only as a prolonged struggle against unfavourable conditions. If a person were to
find the soil clothed with turf on the banks of a stream of water, but on going to some distance on one side of
it, he observed the blades of grass growing thinner and thinner, with intervening patches of sand, until he
entered a desert of sand, he would safely conclude, especially if changes of the same kind were noticed in
other places, that the presence of the water was absolutely necessary to the formation of a thick bed of turf: so
may we conclude, with the same feeling of certainty, that thick beds of coral are formed only at small depths
beneath the surface of the sea.
I have endeavoured to collect every fact, which might either invalidate or corroborate this conclusion. Captain
Moresby, whose opportunities for observation during his survey of the Maldiva and Chagos Archipelagoes
have been unrivalled, informs me, that the upper part or zone of the steep−sided reefs, on the inner and outer
coasts of the atolls in both groups, invariably consists of coral, and the lower parts of sand. At seven or eight
fathoms depth, the bottom is formed, as could be seen through the clear water, of great living masses of coral,
which at about ten fathoms generally stand some way apart from each other, with patches of white sand
between them, and at a little greater depth these patches become united into a smooth steep slope, without any
coral. Captain Moresby, also, informs me in support of his statement, that he found only decayed coral on the
Padua Bank (northern part of the Laccadive group) which has an average depth between twenty−five and
thirty−five fathoms, but that on some other banks in the same group with only ten or twelve fathoms water on
them (for instance, the Tillacapeni bank), the coral was living.
With regard to the coral−reefs in the Red Sea, Ehrenberg has the following passage:−−"The living corals do
not descend there into great depths. On the edges of islets and near reefs, where the depth was small, very
many lived; but we found no more even at six fathoms. The pearl−fishers at Yemen and Massaua asserted that
there was no coral near the pearl−banks at nine fathoms depth, but only sand. We were not able to institute
any more special researches." (Ehrenberg, "Uber die Natur," etc., page 50.) I am, however, assured both by
Captain Moresby and Lieutenant Wellstead, that in the more northern parts of the Red Sea, there are extensive
beds of living coral at a depth of twenty−five fathoms, in which the anchors of their vessels were frequently
entangled. Captain Moresby attributes the less depth, at which the corals are able to live in the places
mentioned by Ehrenberg, to the greater quantity of sediment there; and the situations, where they were
flourishing at the depth of twenty−five fathoms, were protected, and the water was extraordinarily limpid. On
the leeward side of Mauritius where I found the coral growing at a somewhat greater depth than at Keeling
atoll, the sea, owing apparently to its tranquil state, was likewise very clear. Within the lagoons of some of the
Marshall atolls, where the water can be but little agitated, there are, according to Kotzebue, living beds of
coral in twenty−five fathoms. From these facts, and considering the manner in which the beds of clean coral
off Mauritius, Keeling Island, the Maldiva and Chagos atolls, graduated into a sandy slope, it appears very
probable that the depth, at which reef−building polypifers can exist, is partly determined by the extent of
inclined surface, which the currents of the sea and the recoiling waves have the power to keep free from
sediment.
MM. Quoy and Gaimard ("Annales des Sci. Nat." tom. vi.) believe that the growth of coral is confined within
very limited depths; and they state that they never found any fragment of an Astraea (the genus they consider
most efficient in forming reefs) at a depth above twenty−five or thirty feet. But we have seen that in several
places the bottom of the sea is paved with massive corals at more than twice this depth; and at fifteen fathoms
(or twice this depth) off the reefs of Mauritius, the arming was marked with the distinct impression of a living
Astraea. Millepora alcicornis lives in from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0
to 20 fathoms. Captain Moresby has given me a specimen of Sideropora scabra (Porites of Lamarck) brought
up alive from 17 fathoms. Mr. Couthouy ("Remarks on Coral Formations," page 12.) states that he has
dredged up on the Bahama banks considerable masses of Meandrina from 16 fathoms, and he has seen this
coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter, was dredged up alive from 80 fathoms
off Juan Fernandez (latitude 33 deg S.) by Captain P.P. King (I am indebted to Mr. Stokes for having kindly
communicated this fact to me, together with much other valuable information.): this is the most remarkable
fact with which I am acquainted, showing the depth at which a genus of corals often found on reefs, can exist.
We ought, however, to feel less surprise at this fact, as Caryophyllia alone of the lamelliform genera, ranges
far beyond the tropics; it is found in Zetland (Fleming's "British Animals," genus Caryophyllia.) in Latitude
60 deg N. in deep water, and I procured a small species from Tierra del Fuego in Latitude 53 deg S. Captain
Beechey informs me, that branches of pink and yellow coral were frequently brought up from between twenty
and twenty−five fathoms off the Low atolls; and Lieutenant Stokes, writing to me from the N.W. coast of
Australia, says that a strongly branched coral was procured there from thirty fathoms; unfortunately it is not
known to what genera these corals belong.
(I will record in the form of a note all the facts that I have been able to collect on the depths, both within and
without the tropics, at which those corals and corallines can live, which there is no reason to suppose ever
materially aid in the construction of a reef.
(In the following list the name of the Zoophyte is followed by the depth in fathoms, the country and degrees
S. latitude, and the authority. Where no authority is given, the observation is Darwin's own.)
SERTULARIA, 40, Cape Horn 66.
CELLARIA, 40, Cape Horn 66.
CELLARIA, A minute scarlet encrusting species, found living, 190, Keeling Atoll, 12.
CELLARIA, An allied, small stony sub−generic form, 48, St Cruz Riv. 50.
A coral allied to VINCULARIA, with eight rows of cells, 40, Cape Horn.
TUBULIPORA, near to T. patima, 40, Cape Horn.
TUBULIPORA, near to T. patima, 94, East Chiloe 43.
CELLEPORA, several species, and allied sub−generic forms, 40, Cape Horn.
CELLEPORA, several species, and allied sub−generic forms, 40 and 57, Chonos Archipelago 45.
CELLEPORA, several species, and allied sub−generic forms, 48, St Cruz 50.
ESCHARA, 30, Tierra del Fuego 53.
ESCHARA, 48, St Cruz R. 50.
RETEPORA, 40, Cape Horn.
RETEPORA, 100, Cape of Good Hope 34, Quoy and Gaimard, "Ann. Scien. Nat." tome vi., page 284.
MILLEPORA, a strong coral with cylindrical branches, of a pink colour, about two inches high, resembling in
the form of its orifices M. aspera of Lamarck, 94 and 30, E. Chiloe 43, Tierra del Fuego 53.
CORALIUM, 120, Barbary 33 N., Peyssonel in paper read to Royal Society May 1752.
ANTIPATHES, 16, Chonos 45.
GORGONIA (or an allied form), 160, Abrolhos on the coast of Brazil 18, Captain Beechey informed me of
this fact in a letter.
Ellis ("Nat. Hist. of Coralline," page 96) states that Ombellularia was procured in latitude 79 deg N.
STICKING to a LINE from the depth of 236 fathoms; hence this coral either must have been floating loose, or
was entangled in stray line at the bottom. Off Keeling atoll a compound Ascidia (Sigillina) was brought up
from 39 fathoms, and a piece of sponge, apparently living, from 70, and a fragment of Nullipora also
apparently living from 92 fathoms. At a greater depth than 90 fathoms off this coral island, the bottom was
thickly strewed with joints of Halimeda and small fragments of other Nulliporae, but all dead. Captain B.
Allen, R.N., informs me that in the survey of the West Indies it was noticed that between the depth of 10 and
200 fathoms, the sounding lead very generally came up coated with the dead joints of a Halimeda, of which he
showed me specimens. Off Pernambuco, in Brazil, in about twelve fathoms, the bottom was covered with
fragments dead and alive of a dull red Nullipora, and I infer from Roussin's chart, that a bottom of this kind
extends over a wide area. On the beach, within the coral−reefs of Mauritius, vast quantities of fragments of
Nulliporae were piled up. From these facts it appears, that these simply organized bodies are amongst the
most abundant productions of the sea.)
Although the limit of depth, at which each particular kind of coral ceases to exist, is far from being accurately
known; yet when we bear in mind the manner in which the clumps of coral gradually became infrequent at
about the same depth, and wholly disappeared at a greater depth than twenty fathoms, on the slope round
Keeling atoll, on the leeward side of the Mauritius, and at rather less depth, both without and within the atolls
of the Maldiva and Chagos Archipelagoes; and when we know that the reefs round these islands do not differ
from other coral formations in their form and structure, we may, I think, conclude that in ordinary cases,
reef−building polypifers do not flourish at greater depths than between twenty and thirty fathoms.
It has been argued ("Journal of the Royal Geographical Society," 1831, page 218.) that reefs may possibly rise
from very great depths through the means of small corals, first making a platform for the growth of the
stronger kinds. This, however, is an arbitrary supposition: it is not always remembered, that in such cases
there is an antagonist power in action, namely, the decay of organic bodies, when not protected by a covering
of sediment, or by their own rapid growth. We have, moreover, no right to calculate on unlimited time for the
accumulation of small organic bodies into great masses. Every fact in geology proclaims that neither the land,
nor the bed of the sea retain for indefinite periods the same level. As well might it be imagined that the British
Seas would in time become choked up with beds of oysters, or that the numerous small corallines off the
inhospitable shores of Tierra del Fuego would in time form a solid and extensive coral−reef.

Chapter V
Theory of the formation of the different classess of coral - reefs

The atolls of the larger archipelagoes are not formed on submerged craters,
or on banks of sediment.--Immense areas interspersed with atolls.--Their
subsidence.--The effects of storms and earthquakes on atolls.--Recent
changes in their state.--The origin of barrier-reefs and of atolls.--Their
relative forms.--The step-formed ledges and walls round the shores of some
lagoons.--The ring-formed reefs of the Maldiva atolls.--The submerged
condition of parts or of the whole of some annular reefs.--The disseverment
of large atolls.--The union of atolls by linear reefs.--The Great Chagos
Bank.--Objections from the area and amount of subsidence required by the
theory, considered.--The probable composition of the lower parts of atolls.

The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,--those singular rings
of coral-land which rise abruptly out of the unfathomable ocean--and have
passed over, almost unnoticed, the scarcely less wonderful encircling
barrier-reefs.  The theory most generally received on the formation of
atolls, is that they are based on submarine craters; but where can we find
a crater of the shape of Bow atoll, which is five times as long as it is
broad (Plate I., Figure 4); or like that of Menchikoff Island (Plate II.,
Figure 3.), with its three loops, together sixty miles in length; or like
Rimsky Korsacoff, narrow, crooked, and fifty-four miles long; or like the
northern Maldiva atolls, made up of numerous ring-formed reefs, placed on
the margin of a disc,--one of which discs is eighty-eight miles in length,
and only from ten to twenty in breadth?  It is, also, not a little
improbable, that there should have existed as many craters of immense size
crowded together beneath the sea, as there are now in some parts atolls.
But this theory lies under a greater difficulty, as will be evident, when
we consider on what foundations the atolls of the larger archipelagoes
rest: nevertheless, if the rim of a crater afforded a basis at the proper
depth, I am far from denying that a reef like a perfectly characterised
atoll might not be formed; some such, perhaps, now exist; but I cannot
believe in the possibility of the greater number having thus originated.

An earlier and better theory was proposed by Chamisso (Kotzebue's "First
Voyage," volume iii., page 331.); he supposes that as the more massive
kinds of corals prefer the surf, the outer portions, in a reef rising from
a submarine basis, would first reach the surface and consequently form a
ring.  But on this view it must be assumed, that in every case the basis
consists of a flat bank; for if it were conically formed, like a
mountainous mass, we can see no reason why the coral should spring up from
the flanks, instead of from the central and highest parts: considering the
number of the atolls in the Pacific and Indian Oceans, this assumption is
very improbable.  As the lagoons of atolls are sometimes even more than
forty fathoms deep, it must, also, be assumed on this view, that at a depth
at which the waves do not break, the coral grows more vigorously on the
edges of a bank than on its central part; and this is an assumption without
any evidence in support of it.  I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a level
surface some fathoms submerged, having steep sides and being situated in a
deep sea, a reef not to be distinguished from an atoll, might be formed: I
believe some such exist in the West Indies.  But a difficulty of the same
kind with that affecting the crater theory, runners, as we shall presently
see, this view inapplicable to the greater number of atolls.

No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions.  The great
reef which fronts the coast of Australia has been supposed, but without any
special facts, to rest on the edge of a submarine precipice, extending
parallel to the shore.  The origin of the third class or of fringing-reefs
presents, I believe, scarcely any difficulty, and is simply consequent on
the polypifers not growing up from great depths, and their not flourishing
close to gently shelving beaches where the water is often turbid.

What cause, then, has given to atolls and barrier-reefs their
characteristic forms?  Let us see whether an important deduction will not
follow from the consideration of these two circumstances, first, the
reef-building corals flourishing only at limited depths; and secondly, the
vastness of the areas interspersed with coral-reefs and coral-islets, none
of which rise to a greater height above the level of the sea, than that
attained by matter thrown up by the waves and winds.  I do not make this
latter statement vaguely; I have carefully sought for descriptions of every
island in the intertropical seas; and my task has been in some degree
abridged by a map of the Pacific, corrected in 1834 by MM. D'Urville and
Lottin, in which the low islands are distinguished from the high ones (even
from those much less than a hundred feet in height) by being written
without a capital letter; I have detected a few errors in this map,
respecting the height of some of the islands, which will be noticed in the
Appendix, where I treat of coral formations in geographical order.  To the
Appendix, also, I must refer for a more particular account of the data on
which the statements on the next page are grounded.  I have ascertained,
and chiefly from the writings of Cook, Kotzebue, Bellinghausen, Duperrey,
Beechey, and Lutke, regarding the Pacific; and from Moresby (See also
Captain Owen's and Lieutenant Wood's papers in the "Geographical Journal",
on the Maldiva and Laccadive Archipelagoes.  These officers particularly
refer to the lowness of the islets; but I chiefly ground my assertion
respecting these two groups, and the Chagos group, from information
communicated to me by Captain Moresby.) with respect to the Indian Ocean,
that in the following cases the term "low island" strictly means land of
the height commonly attained by matter thrown up by the winds and the waves
of an open sea.  If we draw a line (the plan I have always adopted) joining
the external atolls of that part of the Low Archipelago in which the
islands are numerous, the figure will be a pointed ellipse (reaching from
Hood to Lazaref Island), of which the longer axis is 840 geographical
miles, and the shorter 420 miles; in this space (I find from Mr. Couthouy's
pamphlet (page 58) that Aurora Island is about two hundred feet in height;
it consists of coral-rock, and seems to have been formed by the elevation
of an atoll.  It lies north-east of Tahiti, close without the line bounding
the space coloured dark blue in the map appended to this volume.  Honden
Island, which is situated in the extreme north-west part of the Low
Archipelago, according to measurements made on board the "Beagle", whilst
sailing by, is 114 feet from the SUMMIT OF THE TREES to the water's edge.
This island appeared to resemble the other atolls of the group.) none of
the innumerable islets united into great rings rise above the stated level.
The Gilbert group is very narrow, and 300 miles in length.  In a prolonged
line from this group, at the distance of 240 miles, is the Marshall
Archipelago, the figure of which is an irregular square, one end being
broader than the other; its length is 520 miles, with an average width of
240; these two groups together are 1,040 miles in length, and all their
islets are low.  Between the southern end of the Gilbert and the northern
end of Low Archipelago, the ocean is thinly strewed with islands, all of
which, as far as I have been able to ascertain, are low; so that from
nearly the southern end of the Low Archipelago, to the northern end of the
Marshall Archipelago, there is a narrow band of ocean, more than 4,000
miles in length, containing a great number of islands, all of which are
low.  In the western part of the Caroline Archipelago, there is a space of
480 miles in length, and about 100 broad, thinly interspersed with low
islands.  Lastly, in the Indian Ocean, the archipelago of the Maldivas is
470 miles in length, and 60 in breadth; that of the Laccadives is 150 by
100 miles; as there is a low island between these two groups, they may be
considered as one group of 1,000 miles in length.  To this may be added the
Chagos group of low islands, situated 280 miles distant, in a line
prolonged from the southern extremity of the Maldivas.  This group,
including the submerged banks, is 170 miles in length and 80 in breadth.
So striking is the uniformity in direction of these three archipelagoes,
all the islands of which are low, that Captain Moresby, in one of his
papers, speaks of them as parts of one great chain, nearly 1,500 miles
long.  I am, then, fully justified in repeating, that enormous spaces, both
in the Pacific and Indian Oceans, are interspersed with islands, of which
not one rises above that height, to which the waves and winds in an open
sea can heap up matter.

On what foundations, then, have these reefs and islets of coral been
constructed?  A foundation must originally have been present beneath each
atoll at that limited depth, which is indispensable for the first growth of
the reef-building polypifers.  A conjecture will perhaps be hazarded, that
the requisite bases might have been afforded by the accumulation of great
banks of sediment, which owing to the action of superficial currents (aided
possibly by the undulatory movement of the sea) did not quite reach the
surface,--as actually appears to have been the case in some parts of the
West Indian Sea.  But in the form and disposition of the groups of atolls,
there is nothing to countenance this notion; and the assumption without any
proof, that a number of immense piles of sediment have been heaped on the
floor of the great Pacific and Indian Oceans, in their central parts far
remote from land, and where the dark blue colour of the limpid water
bespeaks its purity, cannot for one moment be admitted.

The many widely-scattered atolls must, therefore, rest on rocky bases.  But
we cannot believe that the broad summit of a mountain lies buried at the
depth of a few fathoms beneath every atoll, and nevertheless throughout the
immense areas above-named, with not one point of rock projecting above the
level of the sea; for we may judge with some accuracy of mountains beneath
the sea, by those on the land; and where can we find a single chain several
hundred miles in length and of considerable breadth, much less several such
chains, with their many broad summits attaining the same height, within
from 120 to 180 feet?  If the data be thought insufficient, on which I have
grounded my belief, respecting the depth at which the reef-building
polypifers can exist, and it be assumed that they can flourish at a depth
of even one hundred fathoms, yet the weight of the above argument is but
little diminished, for it is almost equally improbable, that as many
submarine mountains, as there are low islands in the several great and
widely separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they should be
of the same height within the smaller limit of one or two hundred feet.  So
highly improbable is this supposition, that we are compelled to believe,
that the bases of the many atolls did never at any one period all lie
submerged within the depth of a few fathoms beneath the surface, but that
they were brought into the requisite position or level, some at one period
and some at another, through movements in the earth's crust.  But this
could not have been effected by elevation, for the belief that points so
numerous and so widely separated were successively uplifted to a certain
level, but that not one point was raised above that level, is quite as
improbable as the former supposition, and indeed differs little from it.
It will probably occur to those who have read Ehrenberg's account of the
Reefs of the Red Sea, that many points in these great areas may have been
elevated, but that as soon as raised, the protuberant parts were cut off by
the destroying action of the waves: a moment's reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for the
upheaval and subsequent abrasion of an island would leave a flat disc,
which might become coated with coral, but not a deeply concave surface;
moreover, we should expect to see, in some parts at least, the rock of the
foundation brought to the surface.  If, then, the foundations of the many
atolls were not uplifted into the requisite position, they must of
necessity have subsided into it; and this at once solves every difficulty
(The additional difficulty on the crater hypothesis before alluded to, will
now be evident; for on this view the volcanic action must be supposed to
have formed within the areas specified a vast number of craters, all rising
within a few fathoms of the surface, and not one above it.  The supposition
that the craters were at different times upraised above the surface, and
were there abraded by the surf and subsequently coated by corals, is
subject to nearly the same objections with those given above in this
paragraph; but I consider it superfluous to detail all the arguments
opposed to such a notion.  Chamisso's theory, from assuming the existence
of so many banks, all lying at the proper depth beneath the water, is also
vitally defective.  The same observation applies to an hypothesis of
Lieutenant Nelson's ("Geolog. Trans." volume v., page 122), who supposes
that the ring-formed structure is caused by a greater number of germs of
corals becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster's "Observ." page 151), that lagoon-islands owe their peculiar form
to the instinctive tendencies of the polypifers.  According to this latter
view, the corals on the outer margin of the reef instinctively expose
themselves to the surf in order to afford protection to corals living in
the lagoon, which belong to other genera, and to other families!), for we
may safely infer, from the facts given in the last chapter, that during a
gradual subsidence the corals would be favourably circumstanced for
building up their solid frame works and reaching the surface, as island
after island slowly disappeared.  Thus areas of immense extent in the
central and most profound parts of the great oceans, might become
interspersed with coral-islets, none of which would rise to a greater
height than that attained by detritus heaped up by the sea, and
nevertheless they might all have been formed by corals, which absolutely
required for their growth a solid foundation within a few fathoms of the
surface.

It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas is by
no means improbable.  We have the clearest proof that a movement of this
kind is possible, in the upright trees buried under the strata many
thousand feet in thickness; we have also every reason for believing that
there are now large areas gradually sinking, in the same manner as others
are rising.  And when we consider how many parts of the surface of the
globe have been elevated within recent geological periods, we must admit
that there have been subsidences on a corresponding scale, for otherwise
the whole globe would have swollen.  It is very remarkable that Mr. Lyell
("Principles of Geology," sixth edition, volume iii., page 386.), even in
the first edition of his "Principles of Geology," inferred that the amount
of subsidence in the Pacific must have exceeded that of elevation, from the
area of land being very small relatively to the agents there tending to
form it, namely, the growth of coral and volcanic action.  But it will be
asked, are there any direct proofs of a subsiding movement in those areas,
in which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most difficult,
excepting in countries long civilised, to detect a movement, the tendency
of which is to conceal the part affected.  In barbarous and semi-civilised
nations how long might not a slow movement, even of elevation such as that
now affecting Scandinavia, have escaped attention!

Mr. Williams (Williams's "Narrative of Missionary Enterprise," page 31.)
insists strongly that the traditions of the natives, which he has taken
much pains in collecting, do not indicate the appearance of any new
islands: but on the theory of a gradual subsidence, all that would be
apparent would be, the water sometimes encroaching slowly on the land, and
the land again recovering by the accumulation of detritus its former
extent, and perhaps sometimes the conversion of an atoll with coral islets
on it, into a bare or into a sunken annular reef.  Such changes would
naturally take place at the periods when the sea rose above its usual
limits, during a gale of more than ordinary strength; and the effects of
the two causes would be hardly distinguishable.  In Kotzebue's "Voyage"
there are accounts of islands, both in the Caroline and Marshall
Archipelagoes, which have been partly washed away during hurricanes; and
Kadu, the native who was on board one of the Russian vessels, said "he saw
the sea at Radack rise to the feet of the cocoa-nut trees; but it was
conjured in time."  (Kotzebue's "First Voyage," volume iii., page 168.)  A
storm lately entirely swept away two of the Caroline islands, and converted
them into shoals; it partly, also, destroyed two other islands.  (M.
Desmoulins in "Comptes Rendus," 1840, page 837.)  According to a tradition
which was communicated to Captain Fitzroy, it is believed in the Low
Archipelago, that the arrival of the first ship caused a great inundation,
which destroyed many lives.  Mr. Stutchbury relates, that in 1825, the
western side of Chain Atoll, in the same group, was completely devastated
by a hurricane, and not less than 300 lives lost: "in this instance it was
evident, even to the natives, that the hurricane alone was not sufficient
to account for the violent agitation of the ocean."  ("West of England
Journal", No. I., page 35.)  That considerable changes have taken place
recently in some of the atolls in the Low Archipelago, appears certain from
the case already given of Matilda Island: with respect to Whitsunday and
Gloucester Islands in this same group, we must either attribute great
inaccuracy to their discoverer, the famous circumnavigator Wallis, or
believe that they have undergone a considerable change in the period of
fifty-nine years, between his voyage and that of Captain Beechey's.
Whitsunday Island is described by Wallis as "about four miles long, and
three wide," now it is only one mile and a half long.  The appearance of
Gloucester Island, in Captain Beechey's words (Beechey's "Voyage to the
Pacific," chapter vii., and Wallis's "Voyage in the 'Dolphin'," chapter
iv.), has been accurately described by its discoverer, but its present form
and extent differ materially."  Blenheim reef, in the Chagos group,
consists of a water-washed annular reef, thirteen miles in circumference,
surrounding a lagoon ten fathoms deep: on its surface there were a few
worn patches of conglomerate coral-rock, of about the size of hovels; and
these Captain Moresby considered as being, without doubt, the last remnants
of islets; so that here an atoll has been converted into an atoll-formed
reef.  The inhabitants of the Maldiva Archipelago, as long ago as 1605,
declared, "that the high tides and violent currents were diminishing the
number of the islands"  (See an extract from Pyrard's Voyage in Captain
Owen's paper on the Maldiva Archipelago, in the "Geographical Journal",
volume ii., page 84.): and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to the
present inhabitants.  In such cases, it would be exceedingly difficult to
detect a gradual subsidence of the foundation, on which these mutable
structures rest.

Some of the archipelagoes of low coral-islands are subject to earthquakes:
Captain Moresby informs me that they are frequent, though not very strong,
in the Chagos group, which occupies a very central position in the Indian
Ocean, and is far from any land not of coral formation.  One of the islands
in this group was formerly covered by a bed of mould, which, after an
earthquake, disappeared, and was believed by the residents to have been
washed by the rain through the broken masses of underlying rock; the island
was thus rendered unproductive.  Chamisso (See Chamisso, in Kotzebue's
"First Voyage," volume iii., pages 182 and 136.) states, that earthquakes
are felt in the Marshall atolls, which are far from any high land, and
likewise in the islands of the Caroline Archipelago.  On one of the latter,
namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me,
observed several straight fissures about a foot in width, running for some
hundred yards obliquely across the whole width of the reef.  Fissures
indicate a stretching of the earth's crust, and, therefore, probably
changes in its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably they
have subsided.  In the chapter on Keeling atoll, I attempted to show by
direct evidence, that the island underwent a movement of subsidence, during
the earthquakes lately felt there.

The facts stand thus;--there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the existence
of these reefs and low islets, in such numbers and at such distant points,
is quite inexplicable, excepting on the theory, that the bases on which the
reefs first became attached, slowly and successively sank beneath the level
of the sea, whilst the corals continued to grow upwards.  No positive facts
are opposed to this view, and some general considerations render it
probable.  There is evidence of change in form, whether or not from
subsidence, on some of these coral-islands; and there is evidence of
subterranean disturbances beneath them.  Will then the theory, to which we
have thus been led, solve the curious problem,--what has given to each
class of reef its peculiar form?

(PLATE: WOODCUT NO. 4.

AA--Outer edge of the reef at the level of the sea.

BB--Shores of the island.

A'A'--Outer edge of the reef, after its upward growth during a period of
subsidence.

CC--The lagoon-channel between the reef and the shores of the now encircled
land.

B'B'--The shores of the encircled island.

N.B.--In this, and the following woodcut, the subsidence of the land could
only be represented by an apparent rise in the level of the sea.


PLATE: WOODCUT NO. 5.

A'A'--Outer edges of the barrier-reef at the level of the sea.  The
cocoa-nut trees represent coral-islets formed on the reef.

CC--The lagoon-channel.

B'B'--The shores of the island, generally formed of low alluvial land and
of coral detritus from the lagoon-channel.

A"A"--The outer edges of the reef now forming an atoll.

C'--The lagoon of the newly formed atoll.  According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.)

Let us in imagination place within one of the subsiding areas, an island
surrounded by a "fringing-reef,"--that kind, which alone offers no
difficulty in the explanation of its origin.  Let the unbroken lines and
the oblique shading in the woodcut (No. 4) represent a vertical section
through such an island; and the horizontal shading will represent the
section of the reef.  Now, as the island sinks down, either a few feet at a
time or quite insensibly, we may safely infer from what we know of the
conditions favourable to the growth of coral, that the living masses bathed
by the surf on the margin of the reef, will soon regain the surface.  The
water, however, will encroach, little by little, on the shore, the island
becoming lower and smaller, and the space between the edge of the reef and
the beach proportionately broader.  A section of the reef and island in
this state, after a subsidence of several hundred feet, is given by the
dotted lines: coral-islets are supposed to have been formed on the new
reef, and a ship is anchored in the lagoon-channel.  This section is in
every respect that of an encircling barrier-reef; it is, in fact, a section
taken (The section has been made from the chart given in the "Atlas of the
Voyage of the 'Coquille'."  The scale is .57 of an inch to a mile.  The
height of the island, according to M. Lesson, is 4,026 feet.  The deepest
part of the lagoon-channel is 162 feet; its depth is exaggerated in the
woodcut for the sake of clearness.) east and west through the highest point
of the encircled island of Bolabola; of which a plan is given in Plate I.,
Figure 5.  The same section is more clearly shown in the following woodcut
(No. 5) by the unbroken lines.  The width of the reef, and its slope, both
on the outer and inner side, will have been determined by the growing
powers of the coral, under the conditions (for instance the force of the
breakers and of the currents) to which it has been exposed; and the
lagoon-channel will be deeper or shallower, in proportion to the growth of
the delicately branched corals within the reef, and to the accumulation of
sediment, relatively, also, to the rate of subsidence and the length of the
intervening stationary periods.

It is evident in this section, that a line drawn perpendicularly down from
the outer edge of the new reef to the foundation of solid rock, exceeds by
as many feet as there have been feet of subsidence, that small limit of
depth at which the effective polypifers can live--the corals having grown
up, as the whole sank down, from a basis formed of other corals and their
consolidated fragments.  Thus the difficulty on this head, which before
seemed so great, disappears.

As the space between the reef and the subsiding shore continued to increase
in breadth and depth, and as the injurious effects of the sediment and
fresh water borne down from the land were consequently lessened, the
greater number of the channels, with which the reef in its fringing state
must have been breached, especially those which fronted the smaller
streams, will have become choked up with the growth of coral: on the
windward side of the reef, where the coral grows most vigorously, the
breaches will probably have first been closed.  In barrier-reefs,
therefore, the breaches kept open by draining the tidal waters of the
lagoon-channel, will generally be placed on the leeward side, and they will
still face the mouths of the larger streams, although removed beyond the
influence of their sediment and fresh water;--and this, it has been shown,
is commonly the case.

Referring to the diagram shown above, in which the newly formed barrier-reef
is represented by unbroken lines, instead of by dots as in the former
woodcut, let the work of subsidence go on, and the doubly pointed hill will
form two small islands (or more, according to the number of the hills)
included within one annular reef.  Let the island continue subsiding, and
the coral-reef will continue growing up on its own foundation, whilst the
water gains inch by inch on the land, until the last and highest pinnacle
is covered, and there remains a perfect atoll.  A vertical section of this
atoll is shown in the woodcut by the dotted lines;--a ship is anchored in
its lagoon, but islets are not supposed yet to have been formed on the
reef.  The depth of the lagoon and the width and slope of the reef, will
depend on the circumstances just referred to under barrier-reefs.  Any
further subsidence will produce no change in the atoll, except perhaps a
diminution in its size, from the reef not growing vertically upwards; but
should the currents of the sea act violently upon it, and should the corals
perish on part or on the whole of its margin, changes would result during
subsidence which will be presently noticed.  I may here observe, that a
bank either of rock or of hardened sediment, level with the surface of the
sea, and fringed with living coral, would (if not so small as to allow the
central space to be quickly filled up with detritus) by subsidence be
converted immediately into an atoll, without passing, as in the case of a
reef fringing the shore of an island, through the intermediate form of a
barrier-reef.  If such a bank lay a few fathoms submerged, the simple
growth of the coral (as remarked in the third chapter) without the aid of
subsidence, would produce a structure scarcely to be distinguished from a
true atoll; for in all cases the corals on the outer margin of a reef, from
having space and being freely exposed to the open sea, will grow vigorously
and tend to form a continuous ring whilst the growth of the less massive
kinds on the central expanse, will be checked by the sediment formed there,
and by that washed inwards by the breakers; and as the space becomes
shallower, their growth will, also, be checked by the impurities of the
water, and probably by the small amount of food brought by the enfeebled
currents, in proportion to the surface of living reefs studded with
innumerable craving mouths: the subsidence of a reef based on a bank of
this kind, would give depth to its central expanse or lagoon, steepness to
its flanks, and through the free growth of the coral, symmetry to its
outline:--I may here repeat that the larger groups of atolls in the Pacific
and Indian Oceans cannot be supposed to be founded on banks of this nature.

If, instead of the island in the diagram, the shore of a continent fringed
by a reef had subsided, a great barrier-reef, like that on the north-east
coast of Australia, would have necessarily resulted; and it would have been
separated from the main land by a deep-water channel, broad in proportion
to the amount of subsidence, and to the less or greater inclination of the
neighbouring coast-line.  The effect of the continued subsidence of a great
barrier-reef of this kind, and its probable conversion into a chain of
separate atolls, will be noticed, when we discuss the apparent progressive
disseverment of the larger Maldiva atolls.

We now are able to perceive that the close similarity in form, dimensions,
structure, and relative position (which latter point will hereafter be more
fully noticed) between fringing and encircling barrier-reefs, and between
these latter and atolls, is the necessary result of the transformation,
during subsidence of the one class into the other.  On this view, the three
classes of reefs ought to graduate into each other.  Reefs having
intermediate character between those of the fringing and barrier classes do
exist; for instance, on the south-west coast of Madagascar, a reef extends
for several miles, within which there is a broad channel from seven to
eight fathoms deep, but the sea does not deepen abruptly outside the reef.
Such cases, however, are open to some doubts, for an old fringing-reef,
which had extended itself a little on a basis of its own formation, would
hardly be distinguishable from a barrier-reef, produced by a small amount
of subsidence, and with its lagoon-channel nearly filled up with sediment
during a long stationary period.  Between barrier-reefs, encircling either
one lofty island or several small low ones, and atolls including a mere
expanse of water, a striking series can be shown: in proof of this, I need
only refer to the first plate in this volume, which speaks more plainly to
the eye, than any description could to the ear.  The authorities from which
the charts have been engraved, together with some remarks on them and
descriptive of the plates, are given above.  At New Caledonia (Plate II.,
Figure 5.) the barrier-reefs extend for 150 miles on each side of the
submarine prolongation of the island; and at their northern extremity they
appear broken up and converted into a vast atoll-formed reef, supporting a
few low coral-islets: we may imagine that we here see the effects of
subsidence actually in progress, the water always encroaching on the
northern end of the island, towards which the mountains slope down, and the
reefs steadily building up their massive fabrics in the lines of their
ancient growth.

We have as yet only considered the origin of barrier-reefs and atolls in
their simplest form; but there remain some peculiarities in structure and
some special cases, described in the two first chapters, to be accounted
for by our theory.  These consist--in the inclined ledge terminated by a
wall, and sometimes succeeded by a second ledge with a wall, round the
shores of certain lagoons and lagoon-channels; a structure which cannot, as
I endeavoured to show, be explained by the simple growing powers of the
corals,--in the ring or basin-like forms of the central reefs, as well as
of the separate marginal portions of the northern Maldiva atolls,--in the
submerged condition of the whole, or of parts of certain barrier and
atoll-formed reefs; where only a part is submerged, this being generally to
leeward,--in the apparent progressive disseverment of some of the Maldiva
atolls,--in the existence of irregularly formed atolls, some being tied
together by linear reefs, and others with spurs projecting from them,--and,
lastly, in the structure and origin of the Great Chagos Bank.

STEP-FORMED LEDGES ROUND CERTAIN LAGOONS.

If we suppose an atoll to subside at an extremely slow rate, it is
difficult to follow out the complex results.  The living corals would grow
up on the outer margin; and likewise probably in the gullies and deeper
parts of the bare surface of the annular reef; the water would encroach on
the islets, but the accumulation of fresh detritus might possibly prevent
their entire submergence.  After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would probably
become united with the irregular reefs and banks of sand, which line the
shores of most lagoons.  Should, however, the atoll be carried down by a
more rapid movement, the whole surface of the annular reef, where there was
a foundation of solid matter, would be favourably circumstanced for the
fresh growth of coral; but as the corals grew upwards on its exterior
margin, and the waves broke heavily on this part, the increase of the
massive polypifers on the inner side would be checked from the want of
water.  Consequently, the exterior parts would first reach the surface, and
the new annular reef thus formed on the old one, would have its summit
inclined inwards, and be terminated by a subaqueous wall, formed by the
upward growth of the coral (before being much checked), from the inner edge
of the solid parts of the old reef.  The inner portion of the new reef,
from not having grown to the surface, would be covered by the waters of the
lagoon.  Should a subsidence of the same kind be repeated, the corals would
again grow up in a wall, from all the solid parts of the resunken reef,
and, therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked in
its upward growth, be of less height than the exterior parts, and therefore
would not reach the surface of the lagoon.  In this case the shores of the
lagoon would be surrounded by two inclined ledges, one beneath the other,
and both abruptly terminated by subaqueous cliffs.  (According to Mr.
Couthouy (page 26) the external reef round many atolls descends by a
succession of ledges or terraces.  He attempts, I doubt whether
successfully, to explain this structure somewhat in the same manner as I
have attempted, with respect to the internal ledges round the lagoons of
some atolls.  More facts are wanted regarding the nature both of the
interior and exterior step-like ledges: are all the ledges, or only the
upper ones, covered with living coral?  If they are all covered, are the
kinds different on the ledges according to the depth?  Do the interior and
exterior ledges occur together in the same atolls; if so, what is their
total width, and is the intervening surface-reef narrow, etc.?)

THE RING OR BASIN-FORMED REEFS OF THE NORTHERN MALDIVA ATOLLS.

I may first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards during
subsidence in the same manner as the annular rim; and, therefore, we might
expect that such lagoon-reefs, when not surrounded and buried by an
accumulation of sediment more rapid than the rate of subsidence, would rise
abruptly from a greater depth than that at which the efficient polypifers
can flourish: we see this well exemplified in the small abruptly-sided
reefs, with which the deep lagoons of the Chagos and Southern Maldiva
atolls are studded.  With respect to the ring or basin-formed reefs of the
Northern Maldiva atolls, it is evident, from the perfectly continuous
series which exists that the marginal rings, although wider than the
exterior or bounding reef of ordinary atolls, are only modified portions of
such a reef; it is also evident that the central rings, although wider than
the knolls or reefs which commonly occur in lagoons, occupy their place.
The ring-like structure has been shown to be contingent on the breaches
into the lagoon being broad and numerous, so that all the reefs which are
bathed by the waters of the lagoon are placed under nearly the same
conditions with the outer coast of an atoll standing in the open sea.
Hence the exterior and living margins of these reefs must have been
favourably circumstanced for growing outwards, and increasing beyond the
usual breadth; and they must likewise have been favourably circumstanced
for growing vigorously upwards, during the subsiding movements, to which by
our theory the whole archipelago has been subjected; and subsidence with
this upward growth of the margins would convert the central space of each
little reef into a small lagoon.  This, however, could only take place with
those reefs, which had increased to a breadth sufficient to prevent their
central spaces from being almost immediately filled up with the sand and
detritus driven inwards from all sides: hence it is that few reefs, which
are less than half a mile in diameter, even in the atolls where the
basin-like structure is most strikingly exhibited, include lagoons.  This
remark, I may add, applies to all coral-reefs wherever found.  The
basin-formed reefs of the Maldiva Archipelago may, in fact, be briefly
described, as small atolls formed during subsidence over the separate
portions of large and broken atolls, in the same manner as these latter were
formed over the barrier-reefs, which encircled the islands of a large
archipelago now wholly submerged.

SUBMERGED AND DEAD REEFS.

In the second section of the first chapter, I have shown that there are in
the neighbourhood of atolls, some deeply submerged banks, with level
surfaces; that there are others, less deeply but yet wholly submerged,
having all the characters of perfect atolls, but consisting merely of dead
coral-rock; that there are barrier-reefs and atolls with merely a portion
of their reef, generally on the leeward side, submerged; and that such
portions either retain their perfect outline, or they appear to be quite
effaced, their former place being marked only by a bank, conforming in
outline with that part of the reef which remains perfect.  These several
cases are, I believe, intimately related together, and can be explained by
the same means.  There, perhaps, exist some submerged reefs, covered with
living coral and growing upwards, but to these I do not here refer.

As we see that in those parts of the ocean, where coral-reefs are most
abundant, one island is fringed and another neighbouring one is not
fringed; as we see in the same archipelago, that all the reefs are more
perfect in one part of it than in another, for instance, in the southern
half compared with the northern half of the Maldiva Archipelago, and
likewise on the outer coasts compared with the inner coasts of the atolls
in this same group, which are placed in a double row; as we know that the
existence of the innumerable polypifers forming a reef, depends on their
sustenance, and that they are preyed on by other organic beings; and,
lastly, as we know that some inorganic causes are highly injurious to the
growth of coral, it cannot be expected that during the round of change to
which earth, air, and water are exposed, the reef-building polypifers
should keep alive for perpetuity in any one place; and still less can this
be expected, during the progressive subsidences, perhaps at some periods
more rapid than at others, to which by our theory these reefs and islands
have been subjected and are liable.  It is, then, not improbable that the
corals should sometimes perish either on the whole or on part of a reef; if
on part, the dead portion, after a small amount of subsidence, would still
retain its proper outline and position beneath the water.  After a more
prolonged subsidence, it would probably form, owing to the accumulation of
sediment, only the margin of a flat bank, marking the limits of the former
lagoon.  Such dead portions of reef would generally lie on the leeward side
(Mr. Lyell, in the first edition of his "Principles of Geology," offered a
somewhat different explanation of this structure.  He supposes that there
has been subsidence; but he was not aware that the submerged portions of
reef were in most cases, if not in all, dead; and he attributes the
difference in height in the two sides of most atolls, chiefly to the
greater accumulation of detritus to windward than to leeward.  But as
matter is accumulated only on the backward part of the reef, the front part
would remain of the same height on both sides.  I may here observe that in
most cases (for instance, at Peros Banhos, the Gambier group and the Great
Chagos Bank), and I suspect in all cases, the dead and submerged portions
do not blend or slope into the living and perfect parts, but are separated
from them by an abrupt line.  In some instances small patches of living
reef rise to the surface from the middle of the submerged and dead parts.),
for the impure water and fine sediment would more easily flow out from the
lagoon over this side of the reef, where the force of the breakers is less
than to windward; and therefore the corals would be less vigorous on this
side, and be less able to resist any destroying agent.  It is likewise
owing to this same cause, that reefs are more frequently breached to
leeward by narrow channels, serving as by ship-channels, than to windward.
If the corals perished entirely, or on the greater part of the
circumference of an atoll, an atoll-shaped bank of dead rock, more or less
entirely submerged, would be produced; and further subsidence, together
with the accumulation of sediment, would often obliterate its atoll-like
structure, and leave only a bank with a level surface.

In the Chagos group of atolls, within an area of 160 miles by 60, there are
two atoll-formed banks of dead rock (besides another very imperfect one),
entirely submerged; a third, with merely two or three very small pieces of
living reef rising to the surface; and a fourth, namely, Peros Banhos
(Plate I., Figure 9), with a portion nine miles in length dead and
submerged.  As by our theory this area has subsided, and as there is
nothing improbable in the death, either from changes in the state of the
surrounding sea or from the subsidence being great or sudden, of the corals
on the whole, or on portions of some of the atolls, the case of the Chagos
group presents no difficulty.  So far indeed are any of the above-mentioned
cases of submerged reefs from being inexplicable, that their occurrence
might have been anticipated on our theory, and as fresh atolls are supposed
to be in progressive formation by the subsidence of encircling barrier-reefs,
a weighty objection, namely that the number of atolls must be
increasing infinitely, might even have been raised, if proofs of the
occasional destruction and loss of atolls could not have been adduced.

THE DISSEVERMENT OF THE LARGER MALDIVA ATOLLS.

The apparent progressive disseverment in the Maldiva Archipelago of large
atolls into smaller ones, is, in many respects, an important consideration,
and requires an explanation.  The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting of
detached basin-formed reefs.  The currents of the sea flow across these
atolls, as I am informed by Captain Moresby, with considerable force, and
drift the sediment from side to side during the monsoons, transporting much
of it seaward; yet the currents sweep with greater force round their
flanks.  It is historically known that these atolls have long existed in
their present state; and we can believe, that even during a very slow
subsidence they might thus remain, the central expanse being kept at nearly
its original depth by the accumulation of sediment.  But in the action of
such nicely balanced forces during a progressive subsidence (like that, to
which by our theory this archipelago has been subjected), it would be
strange if the currents of the sea should never make a direct passage
across some one of the atolls, through the many wide breaches in their
margins.  If this were once effected, a deep-water channel would soon be
formed by the removal of the finer sediment, and the check to its further
accumulation; and the sides of the channel would be worn into a slope like
that on the outer coasts, which are exposed to the same force of the
currents.  In fact, a channel precisely like that bifurcating one which
divides Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
formed.  The scattered reefs situated near the borders of the new
ocean-channel, from being favourably placed for the growth of coral, would,
by their extension, tend to produce fresh margins to the dissevered portions;
such a tendency is very evident (as may be seen in the large published
chart) in the elongated reefs on the borders of the two channels
intersecting Mahlos Mahdoo.  Such channels would become deeper with
continued subsidence, and probably from the reefs not growing up
perpendicularly, somewhat broader.  In this case, and more especially if
the channels had been formed originally of considerable breadth, the
dissevered portions would become perfect and distinct atolls, like Ari and
Ross atolls (Plate II., Figure 6), or like the two Nillandoo atolls, which
must be considered as distinct, although related in form and position, and
separated from each other by channels, which though deep have been sounded.
Further subsidence would render such channels unfathomable, and the
dissevered portions would then resemble Phaleedoo and Moluque atolls, or
Mahlos Mahdoo and Horsburgh atolls (Plate II., Figure 4), which are related
to each other in no respect except in proximity and position.  Hence, on
the theory of subsidence, the disseverment of large atolls, which have
imperfect margins (for otherwise their disseverment would be scarcely
possible), and which are exposed to strong currents, is far from being an
improbable event; and the several stages, from close relation to entire
isolation in the atolls of the Maldiva Archipelago, are readily explicable.

We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of nearly
the same dimensions with that of New Caledonia (Plate II., Figure 5), for
if, in imagination, we complete the subsidence of that great island, we
might anticipate from the present broken condition of the northern portion
of the reef, and from the almost entire absence of reefs on the eastern
coast, that the barrier-reef after repeated subsidences, would become
during its upward growth separated into distinct portions; and these
portions would tend to assume an atoll-like structure, from the coral
growing with vigour round their entire circumferences, when freely exposed
to an open sea.  As we have some large islands partly submerged with
barrier-reefs marking their former limits, such as New Caledonia, so our
theory makes it probable that there should be other large islands wholly
submerged; and these, we may now infer, would be surmounted, not by one
enormous atoll, but by several large elongated ones, like the atolls in the
Maldiva group; and these again, during long periods of subsidence, would
sometimes become dissevered into smaller atolls.  I may add, that both in
the Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose, in
such cases, either that two or more encircled islands originally stood
close together, and afforded bases for two or more atolls, or that one
atoll has been dissevered.  From the position, as well as form, of three
atolls in the Caroline Archipelago (the Namourrek and Elato group), which
are placed in an irregular circle, I am strongly tempted to believe that
they have originated by the process of disseverment.  (The same remark is,
perhaps, applicable to the islands of Ollap, Fanadik, and Tamatam in the
Caroline Archipelago, of which charts are given in the atlas of Duperrey's
voyage: a line drawn through the linear reefs and lagoons of these three
islands forms a semicircle.  Consult also, the atlas of Lutke's voyage; and
for the Marshall group that of Kotzebue; for the Gilbert group consult the
atlas of Duperrey's voyage.  Most of the points here referred to may,
however, be seen in Krusenstern's general Atlas of the Pacific.)

IRREGULARLY FORMED ATOLLS.

In the Marshall group, Musquillo atoll consists of two loops united in one
point; and Menchikoff atoll is formed of three loops, two of which (as may
be seen in Figure 3, Plate II.) are connected by a mere ribbon-shaped reef,
and the three together are sixty miles in length.  In the Gilbert group
some of the atolls have narrow strips of reef, like spurs, projecting from
them.  There occur also in parts of the open sea, a few linear and straight
reefs, standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards.  Now, the
upward growth of a barrier-reef which fronted only one side of an island,
or one side of an elongated island with its extremities (of which cases
exist), would produce after the complete subsidence of the land, mere
strips or crescent or hook-formed reefs: if the island thus partially
fronted became divided during subsidence into two or more islands, these
islands would be united together by linear reefs; and from the further
growth of the coral along their shores together with subsidence, reefs of
various forms might ultimately be produced, either atolls united together
by linear reefs, or atolls with spurs projecting from them.  Some, however,
of the more simple forms above specified, might, as we have seen, be
equally well produced by the coral perishing during subsidence on part of
the circumference of an atoll, whilst on the other parts it continued to
grow up till it reached the surface.

THE GREAT CHAGOS BANK.

I have already shown that the submerged condition of the Great Chagos Bank
(Plate II., Figure 1, with its section Figure 2), and of some other banks
in the Chagos group, may in all probability be attributed to the coral
having perished before or during the movements of subsidence, to which this
whole area by our theory has been subjected.  The external rim or upper
ledge (shaded in the chart), consists of dead coral-rock thinly covered
with sand; it lies at an average depth of between five and eight fathoms,
and perfectly resembles in form the annular reef of an atoll.  The banks of
the second level, the boundaries of which are marked by dotted lines in the
chart, lie from about fifteen to twenty fathoms beneath the surface; they
are several miles broad, and terminate in a very steep slope round the
central expanse.  This central expanse I have already described, as
consisting of a level muddy flat between thirty and forty fathoms deep.
The banks of the second level, might at first sight be thought analogous to
the internal step-like ledge of coral-rock which borders the lagoons of
some atolls, but their much greater width, and their being formed of sand,
are points of essential difference.  On the eastern side of the atoll some
of the banks are linear and parallel, resembling islets in a great river,
and pointed directly towards a great breach on the opposite side of the
atoll; these are best seen in the large published chart.  I inferred from
this circumstance, that strong currents sometimes set directly across this
vast bank; and I have since heard from Captain Moresby that this is the
case.  I observed, also, that the channels or breaches through the rim,
were all of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group, and
as I believe into most other large atolls, are not nearly as deep as their
lagoons: for instance at Peros Banhos, the channels are only of the same
depth, namely between ten and twenty fathoms, as the bottom of the lagoon
for a space about a mile and a half in width round its shores, whilst the
central expanse of the lagoon is from thirty-five to forty fathoms deep.
Now, if an atoll during a gradual subsidence once became entirely
submerged, like the Great Chagos Bank, and therefore no longer exposed to
the surf, very little sediment could be formed from it; and consequently
the channels leading into the lagoon from not being filled up with drifted
sand and coral detritus, would continue increasing in depth, as the whole
sank down.  In this case, we might expect that the currents of the open
sea, instead of any longer sweeping round the submarine flanks, would flow
directly through the breaches across the lagoon, removing in their course
the finer sediment, and preventing its further accumulation.  We should
then have the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected by
deep-water channels or breaches, and thus formed into separate marginal
banks; and these would be cut off by steep slopes, overhanging the central
space, worn down by the passage of the oceanic currents.

By these means, I have scarcely any doubt that the Great Chagos Bank has
originated,--a structure which at first appeared to me far more anomalous
than any I had met with.  The process of formation is nearly the same with
that, by which Mahlos Mahdoo had been trisected; but in the Chagos Bank the
channels of the oceanic currents entering at several different quarters,
have united in a central space.

This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged and
dead condition of the whole reef, and the imperfection of the south-east
quarter a mere wreck would probably be left.  The Pitt's Bank, situated not
far southward, appears to be precisely in this state; it consists of a
moderately level, oblong bank of sand, lying from 10 to 20 fathoms beneath
the surface, with two sides protected by a narrow ledge of rock which is
submerged between 5 and 8 fathoms.  A little further south, at about the
same distance as the southern rim of the Great Chagos Bank is from the
northern rim, there are two other small banks with from 10 to 20 fathoms on
them; and not far eastward soundings were struck on a sandy bottom, with
between 110 and 145 fathoms.  The northern portion with its ledge-like
margin, closely resembles any one segment of the Great Chagos Bank, between
two of the deep-water channels, and the scattered banks, southward appear
to be the last wrecks of less perfect portions.

I have examined with care the charts of the Indian and Pacific Oceans, and
have now brought before the reader all the examples, which I have met with,
of reefs differing from the type of the class to which they belong; and I
think it has been satisfactorily shown, that they are all included in our
theory, modified by occasional accidents which might have been anticipated
as probable.  In this course we have seen, that in the lapse of ages
encircling barrier-reefs are occasionally converted into atolls, the name
of atoll being properly applicable, at the moment when the last pinnacle of
encircled land sinks beneath the surface of the sea.  We have, also, seen
that large atolls during the progressive subsidence of the areas in which
they stand, sometimes become dissevered into smaller ones; at other times,
the reef-building polypifers having entirely perished, atolls are converted
into atoll-formed banks of dead rock; and these again through further
subsidence and the accumulation of sediment modified by the force of the
oceanic currents, pass into level banks with scarcely any distinguishing
character.  Thus may the history of an atoll be followed from its first
origin, through the occasional accidents of its existence, to its
destruction and final obliteration.

OBJECTIONS TO THE THEORY OF THE FORMATION OF ATOLLS AND BARRIER-REEFS.

The vast amount of subsidence, both horizontally or in area, and vertically
or in depth, necessary to have submerged every mountain, even the highest,
throughout the immense spaces of ocean interspersed with atolls, will
probably strike most people as a formidable objection to my theory.  But as
continents, as large as the spaces supposed to have subsided, have been
raised above the level of the sea,--as whole regions are now rising, for
instance, in Scandinavia and South America,--and as no reason can be
assigned, why subsidences should not have occurred in some parts of the
earth's crust on as great a scale both in extent and amount as those of
elevation, objections of this nature strike me as of little force.  The
remarkable point is that movements to such an extent should have taken
place within a period, during which the polypifers have continued adding
matter on and above the same reefs.  Another and less obvious objection to
the theory will perhaps be advanced from the circumstance, of the lagoons
within atolls and within barrier-reefs never having become in any one
instance during prolonged subsidences of a greater depth than sixty
fathoms, and seldom more than forty fathoms; but we already admit, if the
theory be worth considering, that the rate of subsidence has not exceeded
that of the upward growth of the coral on the exterior margin; we are,
therefore, only further required to admit, that the subsidence has not
exceeded in rate the filling up of the interior spaces by the growth of the
corals living there, and by the accumulation of sediment.  As this filling
up must take place very slowly within barrier-reefs lying far from the
land, and within atolls which are of large dimensions and which have open
lagoons with very few reefs, we are led to conclude that the subsidence
thus counter-balanced, must have been slow in an extraordinary degree; a
conclusion which accords with our only means, namely, with what is known of
the rate and manner of recent elevatory movements, of judging by analogy
what is the probable rate of subsidence.

In this chapter it has, I think, been shown, that the theory of subsidence,
which we were compelled to receive from the necessity of giving to the
corals, in certain large areas, foundations at the requisite depth,
explains both the normal structure and the less regular forms of those two
great classes of reefs, which have justly excited the astonishment of all
persons who have sailed through the Pacific and Indian Oceans.  But further
to test the truth of the theory, a crowd of questions will occur to the
reader: Do the different kinds of reefs, which have been produced by the
same kind of movement, generally lie within the same areas?  What is their
relation of form and position,--for instance, do adjoining groups of
atolls, and the separate atolls in these groups, bear the same relation to
each other which islands do in common archipelagoes?  Have we reason to
believe, that where there are fringing-reefs, there has not lately been
subsidence; or, for it is almost our only way of ascertaining this point,
are there frequently proofs of recent elevation?  Can we by this means
account for the presence of certain classes of reefs in some large areas,
and their entire absence in others?  Do the areas which have subsided, as
indicated by the presence of atolls and barrier-reefs, and the areas which
have remained stationary or have been upraised, as shown by fringing-reefs,
bear any determinate relation to each other; and are the dimensions of
these areas such as harmonise with the greatness of the subterranean
changes, which, it must be supposed, have lately taken place beneath them?
Is there any connection between the movements thus indicated, and recent
volcanic action?  All these questions ought to receive answers in
accordance with the theory; and if this can be satisfactorily shown, not
only is the theory confirmed, but as deductions, the answers are in
themselves important.  Under this latter point of view, these questions
will be chiefly considered in the following chapter.

(I may take this opportunity of briefly considering the appearances, which
would probably be presented by a vertical and deep section across a coral
formation (referring chiefly to an atoll), formed by the upward growth of
coral during successive subsidences.  This is a subject worthy of
attention, as a means of comparison with ancient coral-strata.  The
circumferential parts would consist of massive species, in a vertical
position, with their interstices filled up with detritus; but this would be
the part most subject to subsequent denudation and removal.  It is useless
to speculate how large a portion of the exterior annular reef would consist
of upright coral, and how much of fragmentary rock, for this would depend
on many contingencies,--such as on the rate of subsidence, occasionally
allowing a fresh growth of coral to cover the whole surface, and on the
breakers having force sufficient to throw fragments over this same space.
The conglomerate which composes the base of the islets, would (if not
removed by denudation together with the exterior reef on which it rests) be
conspicuous from the size of the fragments,--the different degrees in which
they have been rounded,--the presence of fragments of conglomerate torn up,
rounded, and recemented,--and from the oblique stratification.  The corals
which lived in the lagoon-reefs at each successive level, would be
preserved upright, and they would consist of many kinds, generally much
branched.  In this part, however, a very large proportion of the rock (and
in some cases nearly all of it) would be formed of sedimentary matter,
either in an excessively fine, or in a moderately coarse state, and with
the particles almost blended together.  The conglomerate which was formed
of rounded pieces of the branched corals, on the shores of the lagoon,
would differ from that formed on the islets and derived from the outer
coast; yet both might have accumulated very near each other.  I have seen a
conglomerate limestone from Devonshire like a conglomerate now forming on
the shores of the Maldiva atolls.  The stratification taken as a whole,
would be horizontal; but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon (and no doubt
on the external flanks) would probably be divided (as at Keeling atoll and
at Mauritius) by numerous layers dipping at considerable angles in
different directions.  The calcareous sandstone and coral-rock would almost
necessarily contain innumerable shells, echini, and the bones of fish,
turtle, and perhaps of birds; possibly, also, the bones of small saurians,
as these animals find their way to the islands far remote from any
continent.  The large shells of some species of Tridacna would be found
vertically imbedded in the solid rock, in the position in which they lived.
We might expect also to find a mixture of the remains of pelagic and
littoral animals in the strata formed in the lagoon, for pumice and the
seeds of plants are floated from distant countries into the lagoons of many
atolls: on the outer coast of Keeling atoll, near the mouth of the lagoon,
the case of a pelagic Pteropodous animal was brought up on the arming of
the sounding lead.  All the loose blocks of coral on Keeling atoll were
burrowed by vermiform animals; and as every cavity, no doubt, ultimately
becomes filled with spathose limestone, slabs of the rock taken from a
considerable depth, would, if polished, probably exhibit the excavations of
such burrowing animals.  The conglomerate and fine-grained beds of coral-rock
would be hard, sonorous, white and composed of nearly pure calcareous
matter; in some few parts, judging from the specimens at Keeling atoll,
they would probably contain a small quantity of iron.  Floating pumice and
scoriae, and occasionally stones transported in the root of trees (see my
"Journal of Researches," page 549) appear the only sources, through which
foreign matter is brought to coral-formations standing in the open ocean.
The area over which sediment is transported from coral-reefs must be
considerable: Captain Moresby informs me that during the change of
monsoons the sea is discoloured to a considerable distance off the Maldiva
and Chagos atolls.  The sediment of fringing and barrier coral-reefs must
be mingled with the mud, which is brought down from the land, and is
transported seaward through the breaches, which occur in front of almost
every valley.  If the atolls of the larger archipelagoes were upraised, the
bed of the ocean being converted into land, they would form flat-topped
mountains, varying in diameter from a few miles (the smallest atolls being
worn away) to sixty miles; and from being horizontally stratified and of
similar composition, they would, as Mr. Lyell has remarked, falsely appear
as if they had originally been united into one vast continuous mass.  Such
great strata of coral-rock would rarely be associated with erupted volcanic
matter, for this could only take place, as may be inferred from what
follows in the next chapter, when the area, in which they were situated,
commenced to rise, or at least ceased to subside.  During the enormous
period necessary to effect an elevation of the kind just alluded to, the
surface would necessarily be denuded to a great thickness; hence it is
highly improbable that any fringing-reef, or even any barrier-reef, at
least of those encircling small islands, would be preserved.  From this
same cause, the strata which were formed within the lagoons of atolls and
lagoon-channels of barrier-reefs, and which must consist in a large part of
sedimentary matter, would more often be preserved to future ages, than the
exterior solid reef, composed of massive corals in an upright position;
although it is on this exterior part that the present existence and further
growth of atolls and barrier-reefs entirely depend.

CHAPTER VI.--ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION.

(DESCRIPTION OF THE PLATES.

PLATE III.--MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
VOLCANOES.

The principles, on which this map was coloured, are explained in the
beginning of Chapter VI.; and the authorities for each particular spot are
detailed in the Appendix to "Coral Reefs."  The names not printed in upper
case in the Index refer to the Appendix.)

Description of the coloured map.--Proximity of atolls and barrier-reefs.--
Relation in form and position of atolls with ordinary islands.--Direct
evidence of subsidence difficult to be detected.--Proofs of recent
elevation where fringing-reefs occur.--Oscillations of level.--Absence of
active volcanoes in the areas of subsidence.--Immensity of the areas which
have been elevated and have subsided.--Their relation to the present
distribution of the land.--Areas of subsidence elongated, their
intersection and alternation with those of elevation.--Amount and slow rate
of the subsidence.--Recapitulation.

It will be convenient to give here a short account of the appended map
(Plate III.)  [Inasmuch as the coloured map would have proved too costly to
be given in this series, the indications of colour have been replaced by
numbers referring to the dotted groups of reefs, etc.  The author's
original wording, however, is retained in full, as it will be easy to refer
to the map by the numbers, and thus the flow of the narrative is
undisturbed.]: a fuller one, with the data for colouring each spot, is
reserved for the Appendix; and every place there referred to may be found
in the Index.  A larger chart would have been desirable; but, small as the
adjoined one is, it is the result of many months' labour.  I have
consulted, as far as I was able, every original voyage and map; and the
colours were first laid down on charts on a larger scale.  The same blue
colour, with merely a difference in the depth of tint, is used for atolls
or lagoon-islands, and barrier-reefs, for we have seen, that as far as the
actual coral-formation is concerned, they have no distinguishing character.
Fringing-reefs have been coloured red, for between them on the one hand,
and barrier-reefs and atolls on the other, there is an important
distinction with respect to the depth beneath the surface, at which we are
compelled to believe their foundations lie.  The two distinct colours,
therefore, mark two great types of structure.

The DARK BLUE COLOUR [represented by (3) in our plate] represents atolls
and submerged annular reefs, with deep water in their centres.  I have
coloured as atolls, a few low and small coral-islands, without lagoons; but
this has been done only when it clearly appeared that they originally
contained lagoons, since filled up with sediment: when there were not good
grounds for this belief, they have been left uncoloured.

The PALE BLUE COLOUR [represented by (2)] represents barrier-reefs.  The
most obvious character of reefs of this class is the broad and deep-water
moat within the reef: but this, like the lagoons of small atolls, is
liable to become filled up with detritus and with reefs of delicately
branched corals: when, therefore, a reef round the entire circumference of
an island extends very far into a profoundly deep sea, so that it can
hardly be confounded with a fringing-reef which must rest on a foundation
of rock within a small depth, it has been coloured pale blue, although it
does not include a deep-water moat: but this has only been done rarely,
and each case is distinctly mentioned in the Appendix.

The RED COLOUR (4) represents reefs fringing the land quite closely where
the sea is deep, and where the bottom is gently inclined extending to a
moderate distance from it, but not having a deep-water moat or lagoon-like
space parallel to the shore.  It must be remembered that fringing-reefs are
frequently BREACHED in front of rivers and valleys by deepish channels,
where mud has been deposited.  A space of thirty miles in width has been
coloured round or in front of the reefs of each class, in order that the
colours might be conspicuous on the appended map, which is reduced to so
small a scale.

The VERMILLION SPOTS, and streaks (1) represent volcanoes now in action, or
historically known to have been so.  They are chiefly laid down from Von
Buch's work on the Canary Islands; and my reasons for making a few
alterations are given in the note below.

(I have also made considerable use of the geological part of Berghaus'
"Physical Atlas."  Beginning at the eastern side of the Pacific, I have
added to the number of the volcanoes in the southern part of the
Cordillera, and have coloured Juan Fernandez according to observations
collected during the voyage of the "Beagle" ("Geological Transactions,"
volume v., page 601.)  I have added a volcano to Albemarle Island, one of
the Galapagos Archipelago (the author's "Journal of Researches," page 457).
In the Sandwich group there are no active volcanoes, except at Hawaii; but
the Rev. W. Ellis informs me, there are streams of lava apparently modern
on Maui, having a very recent appearance, which can be traced to the
craters whence they flowed.  The same gentleman informs me, that there is
no reason to believe that any active volcano exists in the Society
Archipelago; nor are there any known in the Samoa or Navigator group,
although some of the streams of lava and craters there appear recent.  In
the Friendly group, the Rev. J. Williams says ("Narrative of Missionary
Enterprise," page 29) that Toofoa and Proby Islands are active volcanoes.
I infer from Hamilton's "Voyage in the 'Pandora'" (Page 95), that Proby
Island is synonymous with Onouafou, but I have not ventured to colour it.
There can be no doubt respecting Toofoa, and Captain Edwards (Von Buch,
page 386) found the lava of recent eruption at Amargura still smoking.
Berghaus marks four active volcanoes actually within the Friendly group;
but I do not know on what authority: I may mention that Maurelle describes
Latte as having a burnt-up appearance: I have marked only Toofoa and
Amargura.  South of the New Hebrides lies Matthews Rock, which is drawn and
described as an active crater in the "Voyage of the 'Astrolabe'."  Between
it and the volcano on the eastern side of New Zealand, lies Brimstone
Island, which from the high temperature of the water in the crater, may be
ranked as active (Berghaus "Vorbemerk," II Lief. S. 56).  Malte Brun,
volume xii., page 231, says that there is a volcano near port St. Vincent
in New Caledonia.  I believe this to be an error, arising from a smoke seen
on the OPPOSITE coast by Cook ("Second Voyage," volume ii., page 23) which
smoke went out at night.  The Mariana Islands, especially the northern
ones, contain many craters (see Freycinet's "Hydrog. Descript.") which are
not active.  Von Buch, however, states (page 462) on the authority of La
Peyrouse, that there are no less than seven volcanoes between these islands
and Japan.  Gemelli Creri (Churchill's "Collect." volume iv., page 458),
says there are two active volcanoes in latitude 23 deg 30', and in latitude
24 deg: but I have not coloured them.  From the statements in Beechey's
"Voyage" (page 518, 4to edition) I have coloured one in the northern part
of the Bonin group.  M. S. Julien has clearly made out from Chinese
manuscripts not very ancient ("Comptes Rendus," 1840, page 832), that there
are two active volcanoes on the eastern side of Formosa.  In Torres
Straits, on Cap Island (9 deg 48' S., 142 deg 39' E.) a volcano was seen
burning with great violence in 1793 by Captain Bampton (see Introduction to
Flinders' "Voyage," page 41).  Mr. M'Clelland (Report of Committee for
investigating Coal in India, page 39) has shown that the volcanic band
passing through Barren Island must be extended northwards.  It appears by
an old chart, that Cheduba was once an active volcano (see also "Silliman's
North American Journal", volume xxxviii., page 385).  In Berghaus'
"Physical Atlas," 1840, No. 7 of Geological Part, a volcano on the coast of
Pondicherry is said to have burst forth in 1757.  Ordinaire ("Hist. Nat.
des Volcans," page 218) says that there is one at the mouth of the Persian
Gulf, but I have not coloured it, as he gives no particulars.  A volcano in
Amsterdam, or St. Paul's, in the southern part of the Indian Ocean, has
been seen ("Naut. Mag." 1838, page 842) in action.  Dr. J. Allan, of
Forres, informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and that
the Arabs assured him that they were volcanic, adding that the volcano
burned more during the wet season.  I have marked this as a volcano, though
with some hesitation, on account of the possibility of the flame arising
from gaseous sources.)

The uncoloured coasts consist, first and chiefly, of those, where there are
no coral-reefs, or such small portions as to be quite insignificant.
Secondly, of those coasts where there are reefs, but where the sea is very
shallow, for in this case the reefs generally lie far from the land, and
become very irregular, in their forms: where they have not become
irregular, they have been coloured.  thirdly, if I had the means of
ascertaining the fact, I should not colour a reef merely coating the edges
of a submarine crater, or of a level submerged bank; for such superficial
formations differ essentially, even when not in external appearance, from
reefs whose foundations as well as superficies have been wholly formed by
the growth of coral.  Fourthly, in the Red Sea, and within some parts of
the East Indian Archipelago (if the imperfect charts of the latter can be
trusted), there are many scattered reefs, of small size, represented in the
chart by mere dots, which rise out of deep water: these cannot be arranged
under either of the three classes: in the Red Sea, however, some of these
little reefs, from their position, seem once to have formed parts of a
continuous barrier.  There exist, also, scattered in the open ocean, some
linear and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as they do
not belong to that class, they have not been coloured; they are very few in
number and of insignificant dimensions.  Lastly, some reefs are left
uncoloured from the want of information respecting them, and some because
they are of an intermediate structure between the barrier and fringing
classes.  The value of the map is lessened, in proportion to the number of
reefs which I have been obliged to leave uncoloured, although, in a
theoretical point of view, few of them present any great difficulty: but
their number is not very great, as will be found by comparing the map with
the statements in the Appendix.  I have experienced more difficulty in
colouring fringing-reefs than in colouring barrier-reefs, as the former,
from their much less dimensions, have less attracted the attention of
navigators.  As I have had to seek my information from all kinds of
sources, and often from indirect ones, I do not venture to hope that the
map is free from many errors.  Nevertheless, I trust it will give an
approximately correct view of the general distribution of the coral-reefs
over the whole world (with the exception of some fringing-reefs on the
coast of Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily very
imperfect from the nature of the objects classified, have been adopted by
most voyagers.  I may further remark, that the dark blue colour represents
land entirely composed of coral-rock; the pale blue, land with a wide and
thick border of coral-rock; and the red, a mere narrow fringe of
coral-rock.

Looking now at the map under the theoretical point of view indicated in the
last chapter, the two blue tints signify that the foundations of the reefs
thus coloured have subsided to a considerable amount, at a slower rate than
that of the upward growth of the corals, and that probably in many cases
they are still subsiding.  The red signifies that the shores which support
fringing-reefs have not subsided (at least to any considerable amount, for
the effects of a subsidence on a small scale would in no case be
distinguishable); but that they have remained nearly stationary since the
period when they first became fringed by reefs; or that they are now rising
or have been upraised, with new lines of reefs successively formed on them:
these latter alternatives are obviously implied, as newly formed lines of
shore, after elevations of the land, would be in the same state with
respect to the growth of fringing-reefs, as stationary coasts.  If during
the prolonged subsidence of a shore, coral-reefs grew for the first time on
it, or if an old barrier-reef were destroyed and submerged, and new reefs
became attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of error,
any coast has been coloured wrongly with respect to movement indicated.
Well characterised atolls and encircling barrier-reefs, where several occur
in a group, or a single barrier-reef if of large dimensions, leave scarcely
any doubt on the mind respecting the movement by which they have been
produced; and even a small amount of subsequent elevation is soon betrayed.
The evidence from a single atoll or a single encircling barrier-reef, must
be received with some caution, for the former may possibly be based upon a
submerged crater or bank, and the latter on a submerged margin of sediment,
or of worn-down rock.  From these remarks we may with greater certainty
infer that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or have
been upraised.

ON THE GROUPING OF THE DIFFERENT CLASSES OF REEFS.

Having made these preliminary remarks, I will consider first how far the
grouping of the different kinds of coral-islands and reefs is corroborative
of the truth of the theory.  A glance at the map shows that the reefs,
coloured blue and red, produced under widely different conditions, are not
indiscriminately mixed together.  Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each other;
and this would be the natural result of both having been produced during
the subsidence of the areas in which they stand.  Thus, the largest group
of encircled islands is that of the Society Archipelago; and these islands
are surrounded by atolls, and only separated by a narrow space from the
large group of Low atolls.  In the midst of the Caroline atolls, there are
three fine encircled islands.  The northern point of the barrier-reef of
New Caledonia seems itself, as before remarked, to form a complete large
atoll.  The great Australian barrier is described as including both atolls
and small encircled islands.  Captain King (Sailing directions, appended to
volume ii. of his "Surveying Voyage to Australia.") mentions many
atoll-formed and encircling coral-reefs, some of which lie within the
barrier, and others may be said (for instance between latitude 16 deg and
13 deg) to form part of it.  Flinders ("Voyage to Terra Australis," volume
ii. page 336.) has described an atoll-formed reef in latitude 10 deg, seven
miles long and from one to three broad, resembling a boot in shape, with
apparently very deep water within.  Eight miles westward of this, and
forming part of the barrier, lie the Murray Islands, which are high and are
encircled.  In the Corallian Sea, between the two great barriers of
Australia and New Caledonia, there are many low islets and coral-reefs,
some of which are annular, or horse-shoe shaped.  Observing the smallness
of the scale of the map, the parallels of latitude being nine hundred miles
apart, we see that none of the large groups of reefs and islands supposed
to have been produced by long-continued subsidence, lie near extensive
lines of coast coloured red, which are supposed to have remained stationary
since the growth of their reefs, or to have been upraised and new lines of
reefs formed on them.  Where the red and blue circles do occur near each
other, I am able, in several instances, to show that there have been
oscillations of level, subsidence having preceded the elevation of the red
spots; and elevation having preceded the subsidence of the blue spots: and
in this case the juxtaposition of reefs belonging to the two great types of
structure is little surprising.  We may, therefore, conclude that the
proximity in the same areas of the two classes of reefs, which owe their
origin to the subsidence of the earth's crust, and their separation from
those formed during its stationary or uprising condition, holds good to the
full extent, which might have been anticipated by our theory.

As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores of
one great island, or of several smaller ones; so we might expect that these
rings of coral-rock, like so many rude outline charts, will still retain
some traces of the general form, or at least general range, of the land,
round which they were first modelled.  That this is the case with the
atolls in the Southern Pacific as far as their range is concerned, seems
highly probable, when we observe that the three principal groups are
directed in north-west and south-east lines, and that nearly all the land
in the S. Pacific ranges in this same direction; namely, N. Western
Australia, New Caledonia, the northern half of New Zealand, the New
Hebrides, Saloman, Navigator, Society, Marquesas, and Austral
archipelagoes: in the Northern Pacific, the Caroline atolls abut against
the north-west line of the Marshall atolls, much in the same manner as the
east and west line of islands from Ceram to New Britain do on New Ireland:
in the Indian Ocean the Laccadive and Maldiva atolls extend nearly parallel
to the western and mountainous coast of India.  In most respects, there is
a perfect resemblance with ordinary islands in the grouping of atolls and
in their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand.  The Chagos group is less
elongated than is usual with other groups, and the individual atolls in it
are likewise but little elongated; this is strikingly seen by comparing
them with the neighbouring Maldiva atolls.  In the Marshall and Maldiva
archipelagoes, the atolls are ranged in two parallel lines, like the
mountains in a great double mountain-chain.  Some of the atolls, in the
larger archipelagoes, stand so near to each other, and have such an evident
relationship in form, that they compose little sub-groups: in the Caroline
Archipelago, one such sub-group consists of Pouynipete, a lofty island
encircled by a barrier-reef, and separated by a channel only four miles and
a half wide from Andeema atoll, with a second atoll a little further off.
In all these respects an examination of a series of charts will show how
perfectly groups of atolls resemble groups of common islands.

ON THE DIRECT EVIDENCE OF THE BLUE SPACES IN THE MAP HAVING SUBSIDED DURING
THE UPWARD GROWTH OF THE REEFS SO COLOURED, AND OF THE RED SPACES HAVING
REMAINED STATIONARY, OR HAVING BEEN UPRAISED.

With respect to subsidence, I have shown in the last chapter, that we
cannot expect to obtain in countries inhabited only by semi-civilised
races, demonstrative proofs of a movement, which invariably tends to
conceal its own evidence.  But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance--of a round of decay and renovation--of the last vestiges of
land on some--of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by the
great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some kind
are in progress.  These facts, if not directly connected with subsidence,
as I believe they are, at least show how difficult it would be to discover
proofs of such movement by ordinary means.  At Keeling atoll, however, I
have described some appearances, which seem directly to show that
subsidence did take place there during the late earthquakes.  Vanikoro,
according to Chevalier Dillon (See Captain Dillon's "Voyage in search of La
Peyrouse."  M. Cordier in his "Report on the Voyage of the 'Astrolabe'"
(page cxi., volume i.), speaking of Vanikoro, says the shores are
surrounded by reefs of madrepore, "qu'on assure etre de formation
tout-a-fait moderne."  I have in vain endeavoured to learn some further
particulars about this remarkable passage.  I may here add, that according
to our theory, the island of Pouynipete (Plate I., Figure 7), in the
Caroline Archipelago, being encircled by a barrier-reef, must have
subsided.  In the "New S. Wales Lit. Advert." February 1835 (which I have
seen through the favour of Dr. Lloghtsky), there is an account of this
island (subsequently confirmed by Mr. Campbell), in which it is said, "At
the N.E. end, at a place called Tamen, there are ruins of a town, NOW ONLY
accessible by boats, the waves REACHING TO THE STEPS OF The HOUSES."
Judging from this passage, one would be tempted to conclude that the island
must have subsided, since these houses were built.  I may, also, here
append a statement in Malte Brun (volume ix., page 775, given without any
authority), that the sea gains in an extraordinary manner on the coast of
Cochin China, which lies in front and near the subsiding coral-reefs in the
China Sea: as the coast is granitic, and not alluvial, it is scarcely
possible that the encroachment of the sea can be owing to the washing away
of the land; and if so, it must be due to subsidence.), is often violently
shaken by earthquakes, and there, the unusual depth of the channel between
the shore and the reef,--the almost entire absence of islets on the reef,--
its wall-like structure on the inner side, and the small quantity of low
alluvial land at the foot of the mountains, all seem to show that this
island has not remained long at its present level, with the lagoon-channel
subjected to the accumulation of sediment, and the reef to the wear and
tear of the breakers.  At the Society Archipelago, on the other hand, where
a slight tremor is only rarely felt, the shoaliness of the lagoon-channels
round some of the islands, the number of islets formed on the reefs of
others, and the broad belt of low land at the foot of the mountains,
indicate that, although there must have been great subsidence to have
produced the barrier-reefs, there has since elapsed a long stationary
period.

(Mr. Couthouy states ("Remarks," page 44) that at Tahiti and Eimeo the
space between the reef and the shore has been nearly filled up by the
extension of those coral-reefs, which within most barrier-reefs merely
fringe the land.  From this circumstance, he arrives at the same conclusion
as I have done, that the Society Islands since their subsidence, have
remained stationary during a long period; but he further believes that they
have recently commenced rising, as well as the whole area of the Low
Archipelago.  He does not give any detailed proofs regarding the elevation
of the Society Islands, but I shall refer to this subject in another part
of this chapter.  Before making some further comments, I may observe how
satisfactory it is to me, to find Mr. Couthouy affirming, that "having
personally examined a large number of coral-islands, and also residing
eight months among the volcanic class, having shore and partially
encircling reefs, I may be permitted to state that my own observations have
impressed a conviction of the correctness of the theory of Mr. Darwin."

This gentleman believes, that subsequently to the subsidence by which the
atolls in the Low Archipelago were produced, the whole area has been
elevated to the amount of a few feet; this would indeed be a remarkable
fact; but as far as I am able to judge, the grounds of his conclusion are
not sufficiently strong.  He states that he found in almost every atoll
which he visited, the shores of the lagoon raised from eighteen to thirty
inches above the sea-level, and containing imbedded Tridacnae and corals
standing as they grew; some of the corals were dead in their upper parts,
but below a certain line they continued to flourish.  In the lagoons, also,
he frequently met with clusters of Madrepore, with their extremities
standing from one inch to a foot above the surface of the water.  Now,
these appearances are exactly what I should have expected, without any
subsequent elevation having taken place; and I think Mr. Couthouy has not
borne in mind the indisputable fact, that corals, when constantly bathed by
the surf, can exist at a higher level than in quite tranquil water, as in a
lagoon.  As long, therefore, as the waves continued at low water to break
entirely over parts of the annular reef of an atoll, submerged to a small
depth, the corals and shells attached on these parts might continue living
at a level above the smooth surface of the lagoon, into which the waves
rolled; but as soon as the outer edge of the reef grew up to its utmost
possible height, or if the reef were very broad nearly to that height, the
force of the breakers would be checked, and the corals and shells on the
inner parts near the lagoon would occasionally be left dry, and thus be
partially or wholly destroyed.  Even in atolls, which have not lately
subsided, if the outer margin of the reef continued to increase in breadth
seaward (each fresh zone of corals rising to the same vertical height as at
Keeling atoll), the line where the waves broke most heavily would advance
outwards, and therefore the corals, which when living near the margin, were
washed by the breaking waves during the whole of each tide, would cease
being so, and would therefore be left on the backward part of the reef
standing exposed and dead.  The case of the madrepores in the lagoons with
the tops of their branches exposed, seems to be an analogous fact, to the
great fields of dead but upright corals in the lagoon of Keeling atoll; a
condition of things which I have endeavoured to show, has resulted from the
lagoon having become more and more enclosed and choked up with reefs, so
that during high winds, the rising of the tide (as observed by the
inhabitants) is checked, and the corals, which had formerly grown to the
greatest possible height, are occasionally exposed, and thus are killed:
and this is a condition of things, towards which almost every atoll in the
intervals of its subsidence must be tending.  Or if we look to the state of
an atoll directly after a subsidence of some fathoms, the waves would roll
heavily over the entire circumference of the reef, and the surface of the
lagoon would, like the ocean, never be quite at rest, and therefore the
corals in the lagoon, from being constantly laved by the rippling water,
might extend their branches to a little greater height than they could,
when the lagoon became enclosed and protected.  Christmas atoll (2 deg N.
latitude) which has a very shallow lagoon, and differs in several respects
from most atolls, possibly may have been elevated recently; but its highest
part appears (Couthouy, page 46) to be only ten feet above the sea-level.
The facts of a second class, adduced by Mr. Couthouy, in support of the
alleged recent elevation of the Low Archipelago, are not all (especially
those referring to a shelf of rock) quite intelligible to me; he believes
that certain enormous fragments of rock on the reef, must have been moved
into their present position, when the reef was at a lower level; but here
again the force of the breakers on any inner point of the reef being
diminished by its outward growth without any change in its level, has not,
I think, been borne in mind.  We should, also, not overlook the occasional
agency of waves caused by earthquakes and hurricanes.  Mr. Couthouy further
argues, that since these great fragments were deposited and fixed on the
reef, they have been elevated; he infers this from the greatest amount of
erosion not being near their bases, where they are unceasingly washed by
the reflux of the tides, but at some height on their sides, near the line
of high-water mark, as shown in an accompanying diagram.  My former remark
again applies here, with this further observation, that as the waves have
to roll over a wide space of reef before they reach the fragments, their
force must be greatly increased with the increasing depth of water as the
tide rises, and therefore I should have expected that the chief line of
present erosion would have coincided with the line of high-water mark; and
if the reef had grown outwards, that there would have been lines of erosion
at greater heights.  The conclusion, to which I am finally led by the
interesting observations of Mr. Couthouy is, that the atolls in the Low
Archipelago have, like the Society Islands, remained at a stationary level
for a long period: and this probably is the ordinary course of events,
subsidence supervening after long intervals of rest.)

Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we have
no reason to believe that coral-reefs are of short duration), would not
have remained all this time stationary, but would frequently have undergone
movements of elevation.  This supposition, we shall immediately see, holds
good to a remarkable extent; and although a stationary condition of the
land can hardly ever be open to proof, from the evidence being only
negative, we are, in some degree, enabled to ascertain the correctness of
the parts coloured red on the map, by the direct testimony of upraised
organic remains of a modern date.  Before going into the details on this
head (printed in small type), I may mention, that when reading a memoir on
coral formations by MM. Quoy and Gaimard ("Annales des Sciences Nat." tom.
vi., page 279, etc.) I was astonished to find, for I knew that they had
crossed both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment ended
satisfactorily, when I discovered that, by a strange chance, all the
islands which these eminent naturalists had visited, though several in
number, namely, the Mauritius, Timor, New Guinea, the Mariana, and Sandwich
Archipelagoes, could be shown by their own statements to have been elevated
within a recent geological era.

In the eastern half of the Pacific, the SANDWICH Islands are all fringed,
and almost every naturalist who has visited them, has remarked on the
abundance of elevated corals and shells, apparently identical with living
species.  The Rev. W. Ellis informs me, that he has noticed round several
parts of Hawaii, beds of coral-detritus, about twenty feet above the level
of the sea, and where the coast is low they extend far inland.  Upraised
coral-rock forms a considerable part of the borders of Oahu; and at
Elizabeth Island ("Zoology of Captain Beechey's Voyage," page 176.  See
also MM. Quoy and Gaimard in "Annales de Scien. Nat." tom. vi.) it composes
three strata, each about ten feet thick.  Nihau, which forms the northern,
as Hawaii does the southern end of the group (350 miles in length),
likewise seems to consist of coral and volcanic rocks.  Mr. Couthouy
("Remarks on Coral Formations," page 51.) has lately described with
interesting details, several upraised beaches, ancient reefs with their
surfaces perfectly preserved, and beds of recent shells and corals, at the
islands of Maui, Morokai, Oahu, and Tauai (or Kauai) in this group.  Mr.
Pierce, an intelligent resident at Oahu, is convinced, from changes which
have taken place within his memory, during the last sixteen years, "that
the elevation is at present going forward at a very perceptible rate."  The
natives at Kauai state that the land is there gaining rapidly on the sea,
and Mr. Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.

In the southern part of the Low Archipelago, Elizabeth Island is described
by Captain Beechey (Beechey's "Voyage in the Pacific," page 46, 4to
edition.), as being quite flat, and about eighty feet in height; it is
entirely composed of dead corals, forming a honeycombed, but compact rock.
In cases like this, of an island having exactly the appearance, which the
elevation of any one of the smaller surrounding atolls with a shallow
lagoon would present, one is led to conclude (with little better reason,
however, than the improbability of such small and low fabrics lasting, for
an immense period, exposed to the many destroying agents of nature), that
the elevation has taken place at an epoch not geologically remote.  When
merely the surface of an island of ordinary formation is strewed with
marine bodies, and that continuously, or nearly so, from the beach to a
certain height, and not above that height, it is exceedingly improbable
that such organic remains, although they may not have been specially
examined, should belong to any ancient period.  It is necessary to bear
these remarks in mind, in considering the evidence of the elevatory
movements in the Pacific and Indian Oceans, as it does not often rest on
specific determinations, and therefore should be received with caution.
Six of the COOK AND AUSTRAL Islands (S.W. of the Society group), are
fringed; of these, five were described to me by the Rev. J. Williams, as
formed of coral-rock, associated with some basalt in Mangaia), and the
sixth as lofty and basaltic.  Mangaia is nearly three hundred feet high,
with a level summit; and according to Mr. S. Wilson (Couthouy's "Remarks,"
page 34.) it is an upraised reef; "and there are in the central hollow,
formerly the bed of the lagoon, many scattered patches of coral-rock, some
of them raised to a height of forty feet."  These knolls of coral-rock were
evidently once separate reefs in the lagoon of an atoll.  Mr. Martens, at
Sydney, informed me that this island is surrounded by a terrace-like plain
at about the height of a hundred feet, which probably marks a pause in its
elevation.  From these facts we may infer, perhaps, that the Cook and
Austral Islands have been upheaved at a period probably not very remote.

SAVAGE Island (S.E. of the Friendly group), is about forty feet in height.
Forster ("Observations made during Voyage round the World," page 147.)
describes the plants as already growing out of the dead, but still upright
and spreading trees of coral; and the younger Forster ("Voyage," volume
ii., page 163.) believes that an ancient lagoon is now represented by a
central plain; here we cannot doubt that the elevatory forces have recently
acted.  The same conclusion may be extended, though with somewhat less
certainty, to the islands of the FRIENDLY GROUP, which have been well
described in the second and third voyages of Cook.  The surface of
Tongatabou is low and level, but with some parts a hundred feet high; the
whole consists of coral-rock, "which yet shows the cavities and
irregularities worn into it by the action of the tides."  (Cook's "Third
Voyage" (4to edition), volume i., page 314.)  On Eoua the same appearances
were noticed at an elevation of between two hundred and three hundred feet.
Vavao, also, at the opposite or northern end of the group, consists,
according to the Rev. J. Williams, of coral-rock.  Tongatabou, with its
northern extensive reefs, resembles either an upraised atoll with one half
originally imperfect, or one unequally elevated; and Anamouka, an atoll
equally elevated.  This latter island contains (Ibid., volume i., page
235.) in its centre a salt-water lake, about a mile-and-a-half in diameter,
without any communication with the sea, and around it the land rises
gradually like a bank; the highest part is only between twenty and thirty
feet; but on this part, as well as on the rest of the land (which, as Cook
observes, rises above the height of true lagoon-islands), coral-rock, like
that on the beach, was found.  In the NAVIGATOR ARCHIPELAGO, Mr. Couthouy
("Remarks on Coral-Formations," page 50.) found on Manua many and very
large fragments of coral at the height of eighty feet, "on a steep hill-side,
rising half a mile inland from a low sandy plain abounding in marine
remains."  The fragments were embedded in a mixture of decomposed lava and
sand.  It is not stated whether they were accompanied by shells, or whether
the corals resembled recent species; as these remains were embedded they
possibly may belong to a remote epoch; but I presume this was not the
opinion of Mr. Couthouy.  Earthquakes are very frequent in this
archipelago.

Still proceeding westward we come to the NEW HEBRIDES; on these islands,
Mr. G. Bennett (author of "Wanderings in New South Wales"), informs me he
found much coral at a great altitude, which he considered of recent origin.
Respecting SANTA CRUZ, and the SOLOMON ARCHIPELAGO, I have no information;
but at New Ireland, which forms the northern point of the latter chain,
both Labillardiere and Lesson have described large beds of an apparently
very modern madreporitic rock, with the form of the corals little altered.
The latter author ("Voyage de la 'Coquille'," Part. Zoolog.) states that
this formation composes a newer line of coast, modelled round an ancient
one.  There only remains to be described in the Pacific, that curved line
of fringed islands, of which the MARIANAS form the main part.  Of these
Guam, Rota, Tiniam, Saypan, and some islets farther north, are described by
Quoy and Gaimard (Freycinet's "Voyage autour du Monde."  See also the
"Hydrographical Memoir," page 215.), and Chamisso (Kotzebue's "First
Voyage."), as chiefly composed of madreporitic limestone, which attains a
considerable elevation, and is in several cases worn into successively
rising cliffs: the two former naturalists seem to have compared the corals
and shells with the existing ones, and state that they are of recent
species.  FAIS, which lies in the prolonged line of the Marianas, is the
only island in this part of the sea which is fringed; it is ninety feet
high, and consists entirely of madreporitic rock.  (Lutke's "Voyage,"
volume ii., page 304.)

In the EAST INDIAN ARCHIPELAGO, many authors have recorded proofs of recent
elevation.  M. Lesson (Partie Zoolog., "Voyage de la 'Coquille'.") states,
that near Port Dory, on the north coast of New Guinea, the shores are
flanked, to the height of 150 feet, by madreporitic strata of modern date.
He mentions similar formations at Waigiou, Amboina, Bourou, Ceram, Sonda,
and Timor: at this latter place, MM. Quoy and Gaimard ("Ann. des Scien.
Nat." tom. vi., page 281.) have likewise described the primitive rocks, as
coated to a considerable height with coral.  Some small islets eastward of
Timor are said in Kolff's "Voyage," (translated by Windsor Earl, chapters
vi., vii.) to resemble small coral islets upraised some feet above the sea.
Dr. Malcolmson informs me that Dr. Hardie found in JAVA an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species.  Dr. Jack ("Geolog. Transact." 2nd
series, volume i., page 403.  On the Peninsula of Malacca, in front of
Pinang, 5 deg 30' N., Dr. Ward collected some shells, which Dr. Malcolmson
informs me, although not compared with existing species, had a recent
appearance.  Dr. Ward describes in this neighbourhood ("Trans. Asiat. Soc."
volume xviii., part ii., page 166) a single water-worn rock, with a
conglomerate of sea-shells at its base, situated six miles inland, which,
according to the traditions of the natives, was once surrounded by the sea.
Captain Low has also described (Ibid., part i., page 131) mounds of shells
lying two miles inland on this line of coast.) has described some upraised
shells and corals, apparently recent, on Pulo Nias off SUMATRA; and Marsden
relates in his history of this great island, that the names of many
promontories, show that they were originally islands.  On part of the west
coast of BORNEO and at the SOOLOO Islands, the form of the land, the nature
of the soil, and the water-washed rocks, present appearances ("Notices of
the East Indian Arch." Singapore, 1828, page 6, and Append., page 43.)
(although it is doubtful whether such vague evidence is worthy of mention),
of having recently been covered by the sea; and the inhabitants of the
Sooloo Islands believe that this has been the case.  Mr. Cuming, who has
lately investigated, with so much success, the natural history of the
PHILIPPINES, found near Cabagan, in Luzon, about fifty feet above the level
of the R. Cagayan, and seventy miles from its mouth, a large bed of fossil
shells: these, he informs me, are of the same species with those now
existing on the shores of the neighbouring islands.  From the accounts
given us by Captain Basil Hall and Captain Beechey (Captain B. Hall,
"Voyage to Loo Choo," Append., pages xxi. and xxv.  Captain Beechey's
"Voyage," page 496.) of the lines of inland reefs, and walls of coral-rock
worn into caves, above the present reach of the waves, at the LOO CHOO
Islands, there can be little doubt that they have been upraised at no very
remote period.

Dr. Davy describes the northern province of CEYLON ("Travels in Ceylon,"
page 13.  This madreporitic formation is mentioned by M. Cordier in his
report to the Institute (May 4th, 1839), on the voyage of the "Chevrette",
as one of immense extent, and belonging to the latest tertiary period.) as
being very low, and consisting of a limestone with shells and corals of
very recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man.  There is
also some reason for believing that the western shores of India, north of
Ceylon, have been upraised within the recent period.  (Dr. Benza, in his
"Journey through the N. Circars" (the "Madras Lit. and Scient. Journ."
volume v.) has described a formation with recent fresh-water and marine
shells, occurring at the distance of three or four miles from the present
shore.  Dr. Benza, in conversation with me, attributed their position to a
rise of the land.  Dr. Malcolmson, however (and there cannot be a higher
authority on the geology of India) informs me that he suspects that these
beds may have been formed by the mere action of the waves and currents
accumulating sediment.  From analogy I should much incline to Dr. Benza's
opinion.)  MAURITIUS has certainly been upraised within the recent period,
as I have stated in the chapter on fringing-reefs.  The northern extremity
of MADAGASCAR is described by Captain Owen (Owen's "Africa," volume ii.,
page 37, for Madagascar; and for S. Africa, volume i., pages 412 and 426.
Lieutenant Boteler's narrative contains fuller particulars regarding the
coral-rock, volume i., page 174, and volume ii., pages 41 and 54.  See also
Ruschenberger's "Voyage round the World," volume i., page 60.) as formed of
madreporitic rock, as likewise are the shores and outlying islands along an
immense space of EASTERN AFRICA, from a little north of the equator for
nine hundred miles southward.  Nothing can be more vague than the
expression "madreporitic rock;" but at the same time it is, I think,
scarcely possible to look at the chart of the linear islets, which rise to
a greater height than can be accounted for by the growth of coral, in front
of the coast, from the equator to 2 deg S., without feeling convinced that
a line of fringing-reefs has been elevated at a period so recent, that no
great changes have since taken place on the surface of this part of the
globe.  Some, also, of the higher islands of madreporitic rock on this
coast, for instance Pemba, have very singular forms, which seem to show the
combined effect of the growth of coral round submerged banks, and their
subsequent upheaval.  Dr. Allan informs me that he never observed any
elevated organic remains on the SEYCHELLES, which come under our fringed
class.

The nature of the formations round the shores of the RED SEA, as described
by several authors, shows that the whole of this large area has been
elevated within a very recent tertiary epoch.  A part of this space in the
appended map, is coloured blue, indicating the presence of barrier-reefs:
on which circumstance I shall presently make some remarks.  Ruppell
(Ruppell, "Reise in Abyssinien," Band i., s. 141.) states that the tertiary
formation, of which he has examined the organic remains, forms a fringe
along the shores with a uniform height of from thirty and forty feet from
the mouth of the Gulf of Suez to about latitude 26 deg; but that south of
26 deg, the beds attain only the height of from twelve to fifteen feet.
This, however, can hardly be quite accurate; although possibly there may be
a decrease in the elevation of the shores in the middle parts of the Red
Sea, for Dr. Malcolmson (as he informs me) collected from the cliffs of
Camaran Island (latitude 15 deg 30' S.) shells and corals, apparently
recent, at a height between thirty and forty feet; and Mr. Salt ("Travels
in Abyssinia") describes a similar formation a little southward on the
opposite shore at Amphila.  Moreover, near the mouth of the Gulf of Suez,
although on the coast opposite to that on which Dr. Ruppell says that the
modern beds attain a height of only thirty to forty feet, Mr. Burton
(Lyell's "Principles of Geology," 5th edition, volume iv., page 25.) found
a deposit replete with existing species of shells, at the height of 200
feet.  In an admirable series of drawings by Captain Moresby, I could see
how continuously the cliff-bounded low plains of this formation extended
with a nearly equable height, both on the eastern and western shores.  The
southern coast of Arabia seems to have been subjected to the same elevatory
movement, for Dr. Malcolmson found at Sahar low cliffs containing shells
and corals, apparently of recent species.

The PERSIAN GULF abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured only
some near the mouth; towards the head of the gulf Mr. Ainsworth
(Ainsworth's "Assyria and Babylon," page 217.) says that the land is worn
into terraces, and that the beds contain organic remains of existing forms.
The WEST INDIAN ARCHIPELAGO of "fringed" islands, alone remains to be
mentioned; evidence of an elevation within a late tertiary epoch of nearly
the whole of this great area, may be found in the works of almost all the
naturalists who have visited it.  I will give some of the principal
references in a note.  (On Florida and the north shores of the Gulf of
Mexico, Rogers' "Report to Brit. Assoc." volume iii., page 14.--On the
shores of Mexico, Humboldt, "Polit. Essay on New Spain," volume i., page
62.  (I have also some corroborative facts with respect to the shores of
Mexico.)--Honduras and the Antilles, Lyell's "Principles," 5th edition,
volume iv., page 22.--Santa Cruz and Barbadoes, Prof. Hovey, "Silliman's
Journal", volume xxxv., page 74.--St. Domingo, Courrojolles, "Journ de
Phys." tom. liv., page 106.--Bahamas, "United Service Journal", No. lxxi.,
pages 218 and 224.  Jamaica, De la Beche, "Geol. Man." page 142.--Cuba,
Taylor in "Lond. and Edin. Mag." volume xi., page 17.  Dr. Daubeny also, at
a meeting of the Geolog. Soc., orally described some very modern beds lying
on the N.W. parts of Cuba.  I might have added many other less important
references.)

It is very remarkable on reviewing these details, to observe in how many
instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary period.
It may, however, be objected, that similar proofs of elevation, perhaps,
occur on the coasts coloured blue in our map: but this certainly is not
the case with the few following and doubtful exceptions.

The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on unsatisfactory
evidence, as given in the Appendix) to class the reefs in the middle part,
as barrier-reefs; should, however, the statements prove accurate to the
less height of the tertiary bed in this middle part, compared with the
northern and southern districts, we might well suspect that it had subsided
subsequently to the general elevation by which the whole area has been
upraised.  Several authors (Ellis, in his "Polynesian Researches," was the
first to call attention to these remains (volume i., page 38), and the
tradition of the natives concerning them.  See also Williams, "Nar. of
Missionary Enterprise," page 21; also Tyerman and G. Bennett, "Journal of
Voyage," volume i., page 213; also Mr. Couthouy's "Remarks," page 51; but
this principal fact, namely, that there is a mass of upraised coral on the
narrow peninsula of Tiarubu, is from hearsay evidence; also Mr. Stutchbury,
"West of England Journal," No. i., page 54.  There is a passage in Von
Zach, "Corres. Astronom." volume x., page 266, inferring an uprising at
Tahiti, from a footpath now used, which was formerly impassable; but I
particularly inquired from several native chiefs, whether they knew of any
change of this kind, and they were unanimous in giving me an answer in the
negative.) have stated that they have observed shells and corals high up on
the mountains of the Society Islands,--a group encircled by barrier-reefs,
and, therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found
on the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, "a distinct and regular stratum of semi-fossil
coral."  At Tahiti, however, other naturalists, as well as myself, have
searched in vain at a low level near the coast, for upraised shells or
masses of coral-reef, where if present they could hardly have been
overlooked.  From this fact, I concluded that probably the organic remains
strewed high up on the surface of the land, had originally been embedded in
the volcanic strata, and had subsequently been washed out by the rain.  I
have since heard from the Rev. W. Ellis, that the remains which he met
with, were (as he believes) interstratified with an argillaceous tuff; this
likewise was the case with the shells observed by the Rev. D. Tyerman at
Huaheine.  These remains have not been specifically examined; they may,
therefore, and especially the stratum observed by Mr. Stutchbury at an
immense height, be contemporaneous with the first formation of the Society
Islands, and be of any degree of antiquity; or they may have been deposited
at some subsequent, but probably not very recent, period of elevation; for
if the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral, which certainly is not the case.  Two of the Harvey,
or Cook Islands, namely, Aitutaki and Manouai, are encircled by reefs,
which extend so far from the land, that I have coloured them blue, although
with much hesitation, as the space within the reef is shallow, and the
outline of the land is not abrupt.  These two islands consist of coral-rock;
but I have no evidence of their recent elevation, besides, the
improbability of Mangaia, a fringed island in the same group (but distant
170 miles), having retained its nearly perfect atoll-like structure, during
any immense lapse of time after its upheaval.  The Red Sea, therefore, is
the only area in which we have clear proofs of the recent elevation of a
district, which, by our theory (although the barrier-reefs are there not
well characterised), has lately subsided.  But we have no reason to be
surprised at oscillation, of level of this kind having occasionally taken
place.  There can be scarcely any doubt that Savage, Aurora (Aurora Island
is described by Mr. Couthouy ("Remarks," page 58); it lies 120 miles
north-east of Tahiti; it is not coloured in the appended map, because it does
not appear to be fringed by living reefs.  Mr. Couthouy describes its summit
as "presenting a broad table-land which declines a few feet towards the
centre, where we may suppose the lagoon to have been placed."  It is about
two hundred feet in height, and consists of reef-rock and conglomerate,
with existing species of coral embedded in it.  The island has been
elevated at two successive periods; the cliffs being marked halfway up with

a horizontal water-worn line of deep excavations.  Aurora Island seems
closely to resemble in structure Elizabeth Island, at the southern end of
the Low Archipelago.), and Mangaia Islands, and several of the islands in
the Friendly group, existed originally as atolls, and these have
undoubtedly since been upraised to some height above the level of the sea;
so that by our theory, there has here, also, been an oscillation of level,
--elevation having succeeded subsidence, instead of, as in the middle part
of the Red Sea and at the Harvey Islands, subsidence having probably
succeeded recent elevation.

It is an interesting fact, that Fais, which, from its composition, form,
height, and situation at the western end of the Caroline Archipelago, one
is strongly induced to believe existed before its upheaval as an atoll,
lies exactly in the prolongation of the curved line of the Mariana group,
which we know to be a line of recent elevation.  I may add, that Elizabeth
Island, in the southern part of the Low Archipelago, which seems to have
had the same kind of origin as Fais, lies near Pitcairn Island, the only
one in this part of the ocean which is high, and at the same time not
surrounded by an encircling barrier-reef.

ON THE ABSENCE OF ACTIVE VOLCANOES IN THE AREAS OF SUBSIDENCE, AND ON THEIR
FREQUENT PRESENCE IN THE AREAS OF ELEVATION.

Before making some concluding remarks on the relations of the spaces
coloured blue and red, it will be convenient to consider the position on
our map of the volcanoes historically known to have been in action.  It is
impossible not to be struck, first with the absence of volcanoes in the
great areas of subsidence tinted pale and dark blue,--namely, in the
central parts of the Indian Ocean, in the China Sea, in the sea between the
barriers of Australia and New Caledonia, in the Caroline, Marshall,
Gilbert, and Low Archipelagoes; and, secondly, with the coincidence of the
principal volcanic chains with the parts coloured red, which indicates the
presence of fringing-reefs; and, as we have just seen, the presence in most
cases of upraised organic remains of a modern date.  I may here remark that
the reefs were all coloured before the volcanoes were added to the map, or
indeed before I knew of the existence of several of them.

The volcano in Torres Strait, at the northern point of Australia, is that
which lies nearest to a large subsiding area, although situated 125 miles
within the outer margin of the actual barrier-reef.  The Great Comoro
Island, which probably contains a volcano, is only twenty miles distant
from the barrier-reef of Mohila; Ambil volcano, in the Philippines, is
distant only a little more than sixty miles from the atoll-formed Appoo
reef: and there are two other volcanoes in the map within ninety miles of
circles coloured blue.  These few cases, which thus offer partial
exceptions to the rule, of volcanoes being placed remote from the areas of
subsidence, lie either near single and isolated atolls, or near small
groups of encircled islands; and these by our theory can have, in few
instances, subsided to the same amount in depth or area, as groups of
atolls.  There is not one active volcano within several hundred miles of an
archipelago, or even a small group of atolls.  It is, therefore, a striking
fact that in the Friendly Archipelago, which owes its origin to the
elevation of a group of atolls, two volcanoes, and, perhaps, others are
known to be in action: on the other hand, on several of the encircled
islands in the Pacific, supposed by our theory to have subsided, there are
old craters and streams of lava, which show the effects of past and ancient
eruptions.  In these cases, it would appear as if the volcanoes had come
into action, and had become extinguished on the same spots, according as
the elevating or subsiding movements prevailed.

There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured red
from being fringed by coral-reefs.  Thus I hope to show in a future volume,
that nearly the whole line of the west coast of South America, which forms
the greatest volcanic chain in the world, from near the equator for a space
of between 2,000 and 3,000 miles southward, has undergone an upward
movement during a late geological period.  The islands on the north-western
shores of the Pacific, which form the second greatest volcanic chain, are
very imperfectly known; but Luzon, in the Philippines, and the Loo Choo
Islands, have been recently elevated; and at Kamtschatka (At Sedanka, in
latitude 58 deg N. (Von Buch's "Descrip. des Isles Canaries," page 455).
In a forthcoming part, I shall give the evidence referred to with respect
to the elevation of New Zealand.) there are extensive tertiary beds of
modern date.  Evidence of the same nature, but not very satisfactory, may
be detected in Northern New Zealand where there are two volcanoes.  The
co-existence in other parts of the world of active volcanoes, with upraised
beds of a modern tertiary origin, will occur to every geologist.  (During
the subterranean disturbances which took place in Chile, in 1835, I have
shown ("Geolog. Trans." 2nd Ser., vol. v., page 606) that at the same
moment that a large district was upraised, volcanic matter burst forth at
widely separated points, through both new and old vents.)  Nevertheless,
until it could be shown that volcanoes were inactive, or did not exist in
subsiding areas, the conclusion that their distribution depended on the
nature of the subterranean movements in progress, would have been
hazardous.  But now, viewing the appended map, it may, I think, be
considered as almost established, that volcanoes are often (not necessarily
always) present in those areas where the subterranean motive power has
lately forced, or is now forcing outwards, the crust of the earth, but that
they are invariably absent in those, where the surface has lately subsided
or is still subsiding.  (We may infer from this rule, that in any old
deposit, which contains interstratified beds of erupted matter, there was
at the period, and in the area of its formation, a TENDENCY to an upward
movement in the earth's surface, and certainly no movement of subsidence.)

ON THE RELATIONS OF THE AREAS OF SUBSIDENCE AND ELEVATION.

The immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact.  The existence of continents shows that the areas have been immense
which at some period have been upraised; in South America we may feel sure,
and on the north-western shores of the Indian Ocean we may suspect, that
this rising is either now actually in progress, or has taken place quite
recently.  By our theory, we may conclude that the areas are likewise
immense which have lately subsided, or, judging from the earthquakes
occasionally felt and from other appearances, are now subsiding.  The
smallness of the scale of our map should not be overlooked: each of the
squares on it contains (not allowing for the curvature of the earth)
810,000 square miles.  Look at the space of ocean from near the southern
end of the Low Archipelago to the northern end of the Marshall Archipelago,
a length of 4,500 miles, in which, as far as is known, every island, except
Aurora which lies just without the Low Archipelago, is atoll-formed.  The
eastern and western boundaries of our map are continents, and they are
rising areas: the central spaces of the great Indian and Pacific Oceans,
are mostly subsiding; between them, north of Australia, lies the most
broken land on the globe, and there the rising parts are surrounded and
penetrated by areas of subsidence (I suspect that the Arru and Timor-laut
Islands present an included small area of subsidence, like that of the
China Sea, but I have not ventured to colour them from my imperfect
information, as given in the Appendix.), so that the prevailing movements
now in progress, seem to accord with the actual states of surface of the
great divisions of the world.

The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for the
subsidence of a long, narrow space of the bed of the ocean, including in it
a transverse chain of mountains, surmounted by atolls, would only be marked
on the map by a transverse blue band.  But where a chain of atolls and
barrier-reefs lies in an elongated area, between spaces coloured red, which
therefore have remained stationary or have been upraised, this must have
resulted either from the area of subsidence having originally been
elongated (owing to some tendency in the earth's crust thus to subside), or
from the subsiding area having originally been of an irregular figure, or
as broad as long, and having since been narrowed by the elevation of
neighbouring districts.  Thus the areas, which subsided during the
formation of the great north and south lines of atolls in the Indian
Ocean,--of the east and west line of the Caroline atolls,--and of the
north-west and south-east line of the barrier-reefs of New Caledonia and
Louisiade, must have originally been elongated, or if not so, they must
have since been made elongated by elevations, which we know to belong to a
recent period.

I infer from Mr. Hopkins' researches ("Researches in Physical Geology,"
Transact. Cambridge Phil. Soc., volume vi, part i.), that for the formation
of a long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an elevatory
movement.  Mountain-chains, however, when already formed, although running
in very different directions, it seems (For instance in S. America from
latitude 34 deg, for very many degrees southward there are upraised beds
containing recent species of shells, on both the Atlantic and Pacific side
of the continent, and from the gradual ascent of the land, although with
very unequal slopes, on both sides towards the Cordillera, I think it can
hardly be doubted that the entire width has been upraised in mass within
the recent period.  In this case the two W.N.W. and E.S.E. mountain-lines,
namely the Sierra Ventana and the S. Tapalguen, and the great north and
south line of the Cordillera have been together raised.  In the West Indies
the N. and S. line of the Eastern Antilles, and the E. and W. line of
Jamaica, appear both to have been upraised within the latest geological
period.) may be raised together by a widely-acting force: so, perhaps,
mountain-chains may subside together.  Hence, we cannot tell, whether the
Caroline and Marshall Archipelagoes, two groups of atolls running in
different directions and meeting each other, have been formed by the
subsidence of two areas, or of one large area, including two distinct lines
of mountains.  We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence.  A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.

The existence in many parts of the world of high table-land, proves that
large surfaces have been upraised in mass to considerable heights above the
level of the ocean; although the highest points in almost every country
consist of upturned strata, or erupted matter: and from the immense spaces
scattered with atolls, which indicate that land originally existed there,
although not one pinnacle now remains above the level of the sea, we may
conclude that wide areas have subsided to an amount, sufficient to bury not
only any formerly existing table-land, but even the heights formed by
fractured strata, and erupted matter.  The effects produced on the land by
the later elevatory movements, namely, successively rising cliffs, lines of
erosion, and beds of literal shells and pebbles, all requiring time for
their production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last hundred
feet of rise.  But with reference to the whole vast amount of subsidence,
necessary to have produced the many atolls widely scattered over immense
spaces, it has already been shown (and it is, perhaps, the most interesting
conclusion in this volume), that the movements must either have been
uniform and exceedingly slow, or have been effected by small steps,
separated from each other by long intervals of time, during which the
reef-constructing polypifers were able to bring up their solid frameworks
to the surface.  We have little means of judging whether many considerable
oscillations of level have generally occurred during the elevation of large
tracts; but we know, from clear geological evidence, that this has
frequently taken place; and we have seen on our map, that some of the same
islands have both subsided and been upraised.  I conclude, however, that
most of the large blue spaces, have subsided without many and great
elevatory oscillations, because only a few upraised atolls have been
observed: the supposition that such elevations have taken place, but that
the upraised parts have been worn down by the surf, and thus have escaped
observation, is overruled by the very considerable depth of the lagoons of
all the larger atolls; for this could not have been the case, if they had
suffered repeated elevations and abrasion.  From the comparative
observations made in these latter pages, we may finally conclude, that the
subterranean changes which have caused some large areas to rise, and others
to subside, have acted in a very similar manner.

RECAPITULATION.

In the three first chapters, the principal kinds of coral-reefs were
described in detail, and they were found to differ little, as far as
relates to the actual surface of the reef.  An atoll differs from an
encircling barrier-reef only in the absence of land within its central
expanse; and a barrier-reef differs from a fringing-reef, in being placed
at a much greater distance from the land with reference to the probable
inclination of its submarine foundation, and in the presence of a deep-water
lagoon-like space or moat within the reef.  In the fourth chapter the
growing powers of the reef-constructing polypifers were discussed; and it
was shown, that they cannot flourish beneath a very limited depth.  In
accordance with this limit, there is no difficulty respecting the
foundations on which fringing-reefs are based; whereas, with barrier-reefs
and atolls, there is a great apparent difficulty on this head; in
barrier-reefs from the improbability of the rock of the coast or of banks of
sediment extending, in every instance, so far seaward within the required
depth;--and in atolls, from the immensity of the spaces over which they are
interspersed, and the apparent necessity for believing that they are all
supported on mountain-summits, which although rising very near to the
surface-level of the sea, in no one instance emerge above it.  To escape
this latter most improbable admission, which implies the existence of
submarine chains of mountains of almost the same height, extending over
areas of many thousand square miles, there is but one alternative; namely,
the prolonged subsidence of the foundations, on which the atolls were
primarily based, together with the upward growth of the reef-constructing
corals.  On this view every difficulty vanishes; fringing reefs are thus
converted into barrier-reefs; and barrier-reefs, when encircling islands,
are thus converted into atolls, the instant the last pinnacle of land sinks
beneath the surface of the ocean.

Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;--namely, the wall-like structure
on their inner sides, the basin or ring-like shape both of the marginal and
central reefs in the Maldiva atolls--the union of some atolls as if by a
ribbon--the apparent disseverment of others--and the occurrence, in atolls
as well as in barrier-reefs, of portions of reef, and of the whole of some
reefs, in a dead and submerged state, but retaining the outline of living
reefs.  Thus can be explained the existence of breaches through barrier-reefs
in front of valleys, though separated from them by a wide space of
deep water; thus, also, the ordinary outline of groups of atolls and the
relative forms of the separate atolls one to another; thus can be explained
the proximity of the two kinds of reefs formed during subsidence, and their
separation from the spaces where fringing-reefs abound.  On searching for
other evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it.  On the fringed coasts, however, the presence of
upraised marine bodies of a recent epoch, plainly show, that these coasts,
instead of having remained stationary, which is all that can be directly
inferred from our theory, have generally been elevated.

Finally, when the two great types of structure, namely barrier-reefs and
atolls on the one hand, and fringing-reefs on the other, were laid down in
colours on our map, a magnificent and harmonious picture of the movements,
which the crust of the earth has within a late period undergone, is
presented to us.  We there see vast areas rising, with volcanic matter
every now and then bursting forth through the vents or fissures with which
they are traversed.  We see other wide spaces slowly sinking without any
volcanic outburst, and we may feel sure, that this sinking must have been
immense in amount as well as in area, thus to have buried over the broad
face of the ocean every one of those mountains, above which atolls now
stand like monuments, marking the place of their former existence.
Reflecting how powerful an agent with respect to denudation, and
consequently to the nature and thickness of the deposits in accumulation,
the sea must ever be, when acting for prolonged periods on the land, during
either its slow emergence or subsidence; reflecting, also, on the final
effects of these movements in the interchange of land and ocean-water on
the climate of the earth, and on the distribution of organic beings, I may
be permitted to hope, that the conclusions derived from the study of
coral-formations, originally attempted merely to explain their peculiar
forms, may be thought worthy of the attention of geologists.


APPENDIX.

CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE III.

In the beginning of the last chapter I stated the principles on which the
map is coloured.  There only remains to be said, that it is an exact copy
of one by M. C. Gressier, published by the Depot General de la Marine, in
1835.  The names have been altered into English, and the longitude has been
reduced to that of Greenwich.  The colours were first laid down on accurate
charts, on a large scale.  The data, on which the volcanoes historically
known to have been in action, have been marked with vermillion, were given
in a note to the last chapter.  I will commence my description on the
eastern side of the map, and will describe each group of islands
consecutively, proceeding westward across the Pacific and Indian Oceans,
but ending with the West Indies.

The WESTERN SHORES OF AMERICA appear to be entirely without coral-reefs;
south of the equator the survey of the "Beagle", and north of it, the
published charts show that this is the case.  Even in the Bay of PANAMA,
where corals flourish, there are no true coral-reefs, as I have been
informed by Mr. Lloyd.  There are no coral-reefs in the GALAPAGOS
Archipelago, as I know from personal inspection; and I believe there are
none on the COCOS, REVILLA-GIGEDO, and other neighbouring islands.
CLIPPERTON rock, 10 deg N., 109 deg W., has lately been surveyed by Captain
Belcher; in form it is like the crater of a volcano.  From a drawing
appended to the MS. plan in the Admiralty, it evidently is not an atoll.
The eastern parts of the Pacific present an enormous area, without any
islands, except EASTER, and SALA, and GOMEZ Islands, which do not appear to
be surrounded by reefs.

THE LOW ARCHIPELAGO.

This group consists of about eighty atolls: it will be quite superfluous
to refer to descriptions of each.  In D'Urville and Lottin's chart, one
island (WOLCHONSKY) is written with a capital letter, signifying, as
explained in a former chapter, that it is a high island; but this must be a
mistake, as the original chart by Bellinghausen shows that it is a true
atoll.  Captain Beechey says of the thirty-two groups which he examined (of
the greater number of which I have seen beautiful MS. charts in the
Admiralty), that twenty-nine now contain lagoons, and he believes the other
three originally did.  Bellinghausen (see an account of his Russian voyage,
in the "Biblioth. des Voyages," 1834, page 443) says, that the seventeen
islands which he discovered resembled each other in structure, and he has
given charts on a large scale of all of them.  Kotzebue has given plans of
several; Cook and Bligh mention others; a few were seen during the voyage
of the "Beagle"; and notices of other atolls are scattered through several
publications.  The ACTAEON group in this archipelago has lately been
discovered ("Geographical Journal", volume vii., page 454); it consists of
three small and low islets, one of which has a lagoon.  Another lagoon-island
has been discovered ("Naut. Mag." 1839, page 770), in 22 deg 4' S.,
and 136 deg 20' W.  Towards the S.E. part of the group, there are some
islands of different formation: ELIZABETH Island is described by Beechey
(page 46, 4to edition) as fringed by reefs, at the distance of between two
and three hundred yards; coloured red.  PITCAIRN Island, in the immediate
neighbourhood, according to the same authority, has no reefs of any kind,
although numerous pieces of coral are thrown up on the beach; the sea close
to its shore is very deep (see "Zool. of Beechey's Voyage," page 164); it
is left uncoloured.  GAMBIER Islands (see Plate I., Figure , are
encircled by a barrier-reef; the greatest depth within is thirty-eight
fathoms; coloured pale blue.  AURORA Island, which lies N.E. of Tahiti
close to the large space coloured dark blue in the map, has been already
described in a note (page 71), on the authority of Mr. Couthouy; it is an
upraised atoll, but as it does not appear to be fringed by living reefs, it
is left uncoloured.

The SOCIETY Archipelago is separated by a narrow space from the Low
Archipelago; and in their parallel direction they manifest some relation to
each other.  I have already described the general character of the reefs of
these fine encircled islands.  In the "Atlas of the 'Coquille's' Voyage"
there is a good general chart of the group, and separate plans of some of
the islands.  TAHITI, the largest island in the group, is almost
surrounded, as seen in Cook's chart, by a reef from half a mile to a mile
and a half from the shore, with from ten to thirty fathoms within it.  Some
considerable submerged reefs lying parallel to the shore, with a broad and
deep space within, have lately been discovered ("Naut. Mag." 1836, page
264) on the N.E. coast of the island, where none are laid down by Cook.  At
EIMEO the reef "which like a ring surrounds it, is in some places one or
two miles distant from the shore, in others united to the beach" (Ellis,
"Polynesian Researches," volume i., page 18, 12mo edition).  Cook found
deep water (twenty fathoms) in some of the harbours within the reef.  Mr.
Couthouy, however, states ("Remarks," page 45) that both at Tahiti and
Eimeo, the space between the barrier-reef and the shore, has been almost
filled up,--"a nearly continuous fringing-reef surrounding the island, and
varying from a few yards to rather more than a mile in width, the lagoons
merely forming canals between this and the sea-reef," that is the
barrier-reef.  TAPAMANOA is surrounded by a reef at a considerable distance
from the shore; from the island being small it is breached, as I am informed
by the Rev. W. Ellis, only by a narrow and crooked boat channel.  This is the
lowest island in the group, its height probably not exceeding 500 feet.  A
little way north of Tahiti, the low coral-islets of TETUROA are situated;
from the description of them given me by the Rev. J. Williams (the author
of the "Narrative of Missionary Enterprise"), I should have thought they
had formed a small atoll, and likewise from the description given by the
Rev. D. Tyerman and G. Bennett ("Journal of Voyage and Travels," volume i.,
page 183), who say that ten low coral-islets "are comprehended within one
general reef, and separated from each other by interjacent lagoons;" but as
Mr. Stutchbury ("West of England Journal," volume i., page 54) describes it
as consisting of a mere narrow ridge, I have left it uncoloured.  MAITEA,
eastward of the group, is classed by Forster as a high encircled island;
but from the account given by the Rev. D. Tyerman and G. Bennett (volume
i., page 57) it appears to be an exceedingly abrupt cone, rising from the
sea without any reef; I have left it uncoloured.  It would be superfluous
to describe the northern islands in this group, as they may be well seen in
the chart accompanying the 4to edition of Cook's "Voyages," and in the
"Atlas of the 'Coquille's' Voyage."  MAURUA is the only one of the northern
islands, in which the water within the reef is not deep, being only four
and a half fathoms; but the great width of the reef, stretching three miles
and a half southward of the land (which is represented in the drawing in
the "Atlas of the 'Coquille's' Voyage" as descending abruptly to the water)
shows, on the principle explained in the beginning of the last chapter,
that it belongs to the barrier class.  I may here mention, from information
communicated to me by the Rev. W. Ellis, that on the N.E. side of HUAHEINE
there is a bank of sand, about a quarter of a mile wide, extending parallel
to the shore, and separated from it by an extensive and deep lagoon; this
bank of sand rests on coral-rock, and undoubtedly was originally a living
reef.  North of Bolabola lies the atoll of TOUBAI (Motou-iti of the
"'Coquille's' Atlas") which is coloured dark blue; the other islands,
surrounded by barrier-reefs, are pale blue; three of them are represented
in Figures 3, 4, and 5, in Plate I.  There are three low coral-groups lying
a little E. of the Society Archipelago, and almost forming part of it,
namely BELLINGHAUSEN, which is said by Kotzebue ("Second Voyage," volume
ii., page 255), to be a lagoon-island; MOPEHA, which, from Cook's
description ("Second Voyage," book iii., chapter i.), no doubt is an atoll;
and the SCILLY Islands, which are said by Wallis ("Voyage," chapter ix.) to
form a GROUP of LOW islets and shoals, and, therefore, probably, they
compose an atoll: the two former have been coloured blue, but not the
latter.

MENDANA OR MARQUESAS GROUP.

These islands are entirely without reefs, as may be seen in Krusenstern's
Atlas, making a remarkable contrast with the adjacent group of the Society
Islands.  Mr. F.D. Bennett has given some account of this group, in the
seventh volume of the "Geographical Journal".  He informs me that all the
islands have the same general character, and that the water is very deep
close to their shores.  He visited three of them, namely, DOMINICANA,
CHRISTIANA, and ROAPOA; their beaches are strewed with rounded masses of
coral, and although no regular reefs exist, yet the shore is in many places
lined by coral-rock, so that a boat grounds on this formation.  Hence these
islands ought probably to come within the class of fringed islands and be
coloured red; but as I am determined to err on the cautious side, I have
left them uncoloured.

COOK OR HARVEY AND AUSTRAL ISLAND.

PALMERSTON Island is minutely described as an atoll by Captain Cook during
his voyage in 1774; coloured blue.  AITUTAKI was partially surveyed by the
"Beagle" (see map accompanying "Voyages of 'Adventure' and 'Beagle'"); the
land is hilly, sloping gently to the beach; the highest point is 360 feet;
on the southern side the reef projects five miles from the land: off this
point the "Beagle" found no bottom with 270 fathoms: the reef is
surmounted by many low coral-islets.  Although within the reef the water is
exceedingly shallow, not being more than a few feet deep, as I am informed
by the Rev. J. Williams, nevertheless, from the great extension of this
reef into a profoundly deep ocean, this island probably belongs, on the
principle lately adverted to, to the barrier class, and I have coloured it
pale blue; although with much hesitation.--MANOUAI or HARVEY Island.  The
highest point is about fifty feet: the Rev. J. Williams informs me that
the reef here, although it lies far from the shore, is less distant than at
Aitutaki, but the water within the reef is rather deeper: I have also
coloured this pale blue with many doubts.--Round MITIARO Island, as I am
informed by Mr. Williams, the reef is attached to the shore; coloured red.
--MAUKI or Maouti; the reef round this island (under the name of Parry
Island, in the "Voyage of H.M.S. 'Blonde'," page 209) is described as a
coral-flat, only fifty yards wide, and two feet under water.  This
statement has been corroborated by Mr. Williams, who calls the reef
attached; coloured red.--AITU, or Wateeo; a moderately elevated hilly
island, like the others of this group.  The reef is described in Cook's
"Voyage," as attached to the shore, and about one hundred yards wide;
coloured red.--FENOUA-ITI; Cook describes this island as very low, not more
than six or seven feet high (volume i., book ii., chapter iii, 1777); in
the chart published in the "'Coquille's' Atlas," a reef is engraved close
to the shore: this island is not mentioned in the list given by Mr.
Williams (page 16) in the "Narrative of Missionary Enterprise;" nature
doubtful.  As it is so near Atiu, it has been unavoidably coloured red.--
RAROTONGA; Mr. Williams informs me that it is a lofty basaltic island with
an attached reef; coloured red.--There are three islands, ROUROUTI,
ROXBURGH, and HULL, of which I have not been able to obtain any account,
and have left them uncoloured.  Hull Island, in the French chart, is
written with small letters as being low.--MANGAIA; height about three
hundred feet; "the surrounding reef joins the shore" (Williams,
"Narrative," page 18); coloured red.--RIMETARA; Mr. Williams informs me
that the reef is rather close to the shore; but, from information given me
by Mr. Ellis, the reef does not appear to be quite so closely attached to
it as in the foregoing cases: the island is about three hundred feet high
("Naut. Mag." 1839, page 738); coloured red.--RURUTU; Mr. Williams and Mr.
Ellis inform me that this island has an attached reef; coloured red.  It is
described by Cook under the name of Oheteroa: he says it is not
surrounded, like the neighbouring islands by a reef; he must have meant a
distant reef.--TOUBOUAI; in Cook's chart ("Second Voyage," volume ii., page
2) the reef is laid down in part one mile, and in part two miles from the
shore.  Mr. Ellis ("Polynes. Res." volume iii., page 381) says the low land
round the base of the island is very extensive; and this gentleman informs
me that the water within the reef appears deep; coloured blue.--RAIVAIVAI,
or Vivitao; Mr. Williams informs me that the reef is here distant: Mr.
Ellis, however, says that this is certainly not the case on one side of the
island; and he believes that the water within the reef is not deep; hence I
have left it uncoloured.--LANCASTER Reef, described in "Naut. Mag." 1833
(page 693), as an extensive crescent-formed coral-reef.  I have not
coloured it.--RAPA, or Oparree; from the accounts given of it by Ellis and
Vancouver, there does not appear to be any reef.--I. DE BASS is an
adjoining island, of which I cannot find any account.--KEMIN Island;
Krusenstern seems hardly to know its position, and gives no further
particulars.

ISLANDS BETWEEN THE LOW AND GILBERT ARCHIPELAGOES.

CAROLINE Island (10 deg S., 150 deg W.) is described by Mr. F.D. Bennett
("Geographical Journal", volume vii., page 225) as containing a fine
lagoon; coloured blue.--FLINT Island (11 deg S., 151 deg W.); Krusenstern
believes that it is the same with Peregrino, which is described by Quiros
(Burney's "Chron. Hist." volume ii., page 283) as "a cluster of small
islands connected by a reef, and forming a lagoon in the middle;" coloured
blue.--WOSTOCK is an island a little more than half a mile in diameter, and
apparently quite flat and low, and was discovered by Bellinghausen; it is
situated a little west of Caroline Island, but it is not placed on the
French charts; I have not coloured it, although I entertain little doubt
from the chart of Bellinghausen, that it originally contained a small
lagoon.--PENRHYN Island (9 deg S., 158 deg W.); a plan of it in the "Atlas
of the First Voyage" of Kotzebue, shows that it is an atoll; blue.--
SLARBUCK Island (5 deg S., 156 deg W.) is described in Byron's "Voyage in
the 'Blonde'" (page 206) as formed of a flat coral-rock, with no trees; the
height not given; not coloured.--MALDEN Island (4 deg S., 154 deg W.); in
the same voyage (page 205) this island is said to be of coral formation,
and no part above forty feet high; I have not ventured to colour it,
although, from being of coral-formation, it is probably fringed; in which
case it should be red.--JARVIS, or BUNKER Island (0 deg 20' S., 160 deg W.)
is described by Mr. F.D. Bennett ("Geographical Journal", volume vii., page
227) as a narrow, low strip of coral-formation; not coloured.--BROOK, is a
small low island between the two latter; the position, and perhaps even the
existence of it is doubtful; not coloured.--PESCADO and HUMPHREY Islands; I
can find out nothing about these islands, except that the latter appears to
be small and low; not coloured.--REARSON, or Grand Duke Alexander's (10 S.,
161 deg W.); an atoll, of which a plan is given by Bellinghausen; blue.--
SOUVOROFF Islands (13 deg S., 163 deg W.); Admiral Krusenstern, in the most
obliging manner, obtained for me an account of these islands from Admiral
Lazareff, who discovered them.  They consist of five very low islands of
coral-formation, two of which are connected by a reef, with deep water
close to it.  They do not surround a lagoon, but are so placed that a line
drawn through them includes an oval space, part of which is shallow; these
islets, therefore, probably once (as is the case with some of the islands
in the Caroline Archipelago) formed a single atoll; but I have not coloured
them.--DANGER Island (10 deg S., 166 deg W.); described as low by Commodore
Byron, and more lately surveyed by Bellinghausen; it is a small atoll with
three islets on it; blue.--CLARENCE Island (9 deg S., 172 deg W.);
discovered in the "Pandora" (G. Hamilton's "Voyage," page 75): it is said,
"in running along the land, we saw several canoes crossing the LAGOONS;" as
this island is in the close vicinity of other low islands, and as it is
said, that the natives make reservoirs of water in old cocoa-nut trees
(which shows the nature of the land), I have no doubt it is an atoll, and
have coloured it blue.  YORK Island (8 deg S., 172 deg W.) is described by
Commodore Byron (chapter x. of his "Voyage") as an atoll; blue.--SYDNEY
Island (4 deg S., 172 deg W.) is about three miles in diameter, with its
interior occupied by a lagoon (Captain Tromelin, "Annal. Marit." 1829, page
297); blue.--PHOENIX Island (4 deg S., 171 deg W.) is nearly circular, low,
sandy, not more than two miles in diameter, and very steep outside
(Tromelin, "Annal. Marit." 1829, page 297); it may be inferred that this
island originally contained a lagoon, but I have not coloured it.--NEW
NANTUCKET (0 deg 15' N., 174 deg W.).  From the French chart it must be a
low island; I can find nothing more about it or about MARY Island; both
uncoloured.--GARDNER Island (5 deg S., 174 deg W.) from its position is
certainly the same as KEMIN Island described (Krusenstern, page 435, Appen.
to Mem., published 1827) as having a lagoon in its centre; blue.

ISLANDS SOUTH OF THE SANDWICH ARCHIPELAGO.

CHRISTMAS Island (2 deg N., 157 deg W.).  Captain Cook, in his "Third
Voyage" (Volume ii., chapter x.), has given a detailed account of this
atoll.  The breadth of the islets on the reef is unusually great, and the
sea near it does not deepen so suddenly as is generally the case.  It has
more lately been visited by Mr. F.D. Bennett ("Geographical Journal,"
volume vii., page 226); and he assures me that it is low and of
coral-formation: I particularly mention this, because it is engraved with a
capital letter, signifying a high island, in D'Urville and Lottin's chart.
Mr. Couthouy, also, has given some account of it ("Remarks," page 46) from
the Hawaiian "Spectator"; he believes it has lately undergone a small
elevation, but his evidence does not appear to me satisfactory; the deepest
part of the lagoon is said to be only ten feet; nevertheless, I have
coloured it blue.--FANNING Island (4 deg N., 158 deg W.) according to
Captain Tromelin ("Ann. Maritim." 1829, page 283), is an atoll: his
account as observed by Krusenstern, differs from that given in Fanning's
"Voyage" (page 224), which, however, is far from clear; coloured blue.--
WASHINGTON Island (4 deg N., 159 deg W.) is engraved as a low island in
D'Urville's chart, but is described by Fanning (page 226) as having a much
greater elevation than Fanning Island, and hence I presume it is not an
atoll; not coloured.--PALMYRA Island (6 deg N., 162 deg W.) is an atoll
divided into two parts (Krusenstern's "Mem. Suppl." page 50, also Fanning's
"Voyage," page 233); blue.--SMYTH'S or Johnston's Islands (17 deg N., 170
deg W.).  Captain Smyth, R.N., has had the kindness to inform me that they
consist of two very low, small islands, with a dangerous reef off the east
end of them.  Captain Smyth does not recollect whether these islets,
together with the reef, surrounded a lagoon; uncoloured.

SANDWICH ARCHIPELAGO.

HAWAII; in the chart in Freycinet's "Atlas," small portions of the coast
are fringed by reefs; and in the accompanying "Hydrog. Memoir," reefs are
mentioned in several places, and the coral is said to injure the cables.
On one side of the islet of Kohaihai there is a bank of sand and coral with
five feet water on it, running parallel to the shore, and leaving a channel
of about fifteen feet deep within.  I have coloured this island red, but it
is very much less perfectly fringed than others of the group.--MAUI; in
Freycinet's chart of the anchorage of Raheina, two or three miles of coast
are seen to be fringed; and in the "Hydrog. Memoir," "banks of coral along
shore" are spoken of.  Mr. F.D. Bennett informs me that the reefs, on an
average, extend about a quarter of a mile from the beach; the land is not
very steep, and outside the reefs the sea does not become deep very
suddenly; coloured red.--MOROTOI, I presume, is fringed: Freycinet speaks
of the breakers extending along the shore at a little distance from it.
From the chart, I believe it is fringed; coloured red.--OAHU; Freycinet, in
his "Hydrog. Memoir," mentions some of the reefs.  Mr. F.D. Bennett informs
me that the shore is skirted for forty or fifty miles in length.  There is
even a harbour for ships formed by the reefs, but it is at the mouth of a
valley; red.--ATOOI, in La Peyrouse's charts, is represented as fringed by
a reef, in the same manner as Oahu and Morotoi; and this, as I have been
informed by Mr. Ellis, on part at least of the shore, is of coral-formation:
the reef does not leave a deep channel within; red.--ONEEHOW;
Mr. Ellis believes that this island is also fringed by a coral-reef:
considering its close proximity to the other islands, I have ventured to
colour it red.  I have in vain consulted the works of Cook, Vancouver, La
Peyrouse, and Lisiansky, for any satisfactory account of the small islands
and reefs, which lie scattered in a N.W. line prolonged from the Sandwich
group, and hence have left them uncoloured, with one exception; for I am
indebted to Mr. F.D. Bennett for informing me of an atoll-formed reef, in
latitude 28 deg 22', longitude 178 deg 30' W., on which the "Gledstanes"
was wrecked in 1837.  It is apparently of large size, and extends in a N.W.
and S.E. line: very few islets have been formed on it.  The lagoon seems
to be shallow; at least, the deepest part which was surveyed was only three
fathoms.  Mr. Couthouy ("Remarks," page 38) describes this island under the
name of OCEAN island.  Considerable doubts should be entertained regarding
the nature of a reef of this kind, with a very shallow lagoon, and standing
far from any other atoll, on account of the possibility of a crater or flat
bank of rock lying at the proper depth beneath the surface of the water,
thus affording a foundation for a ring-formed coral-reef.  I have, however,
thought myself compelled, from its large size and symmetrical outline, to
colour it blue.

SAMOA OR NAVIGATOR GROUP.

Kotzebue, in his "Second Voyage," contrasts the structure of these islands
with many others in the Pacific, in not being furnished with harbours for
ships, formed by distant coral-reefs.  The Rev. J. Williams, however,
informs me, that coral-reefs do occur in irregular patches on the shores of
these islands; but that they do not form a continuous band, as round
Mangaia, and other such perfect cases of fringed islands.  From the charts
accompanying La Peyrouse's "Voyage," it appears that the north shore of
SAVAII, MAOUNA, OROSENGA, and MANUA, are fringed by reefs.  La Peyrouse,
speaking of Maouna (page 126), says that the coral-reef surrounding its
shores, almost touches the beach; and is breached in front of the little
coves and streams, forming passages for canoes, and probably even for
boats.  Further on (page 159), he extends the same observation to all the
islands which he visited.  Mr. Williams in his "Narrative," speaks of a
reef going round a small island attached to OYOLAVA, and returning again to
it: all these islands have been coloured red.--A chart of ROSE Island, at
the extreme west end of the group, is given by Freycinet, from which I
should have thought that it had been an atoll; but according to Mr.
Couthouy ("Remarks," page 43), it consists of a reef, only a league in
circuit, surmounted by a very few low islets; the lagoon is very shallow,
and is strewed with numerous large boulders of volcanic rock.  This island,
therefore, probably consists of a bank of rock, a few feet submerged, with
the outer margin of its upper surface fringed with reefs; hence it cannot
be properly classed with atolls, in which the foundations are always
supposed to lie at a depth, greater than that at which the reef-constructing
polypifers can live; not coloured.

BEVERIDGE Reef, 20 deg S., 167 deg W., is described in the "Naut. Mag."
(May 1833, page 442) as ten miles long in a N. and S. line, and eight wide;
"in the inside of the reef there appears deep water;" there is a passage
near the S.W. corner: this therefore seems to be a submerged atoll, and is
coloured blue.

SAVAGE Island, 19 deg S., 170 deg W., has been described by Cook and
Forster.  The younger Forster (volume ii., page 163) says it is about forty
feet high: he suspects that it contains a low plain, which formerly was
the lagoon.  The Rev. J. Williams informs me that the reef fringing its
shores, resembles that round Mangaia; coloured red.

FRIENDLY ARCHIPELAGO.

PYLSTAART Island.  Judging from the chart in Freycinet's "Atlas," I should
have supposed that it had been regularly fringed; but as nothing is said in
the "Hydrog. Memoir" (or in the "Voyage" of Tasman, the discoverer) about
coral-reefs, I have left it uncoloured.--TONGATABOU: In the "Atlas of the
Voyage of the 'Astrolabe'," the whole south side of the island is
represented as narrowly fringed by the same reef which forms an extensive
platform on the northern side.  The origin of this latter reef, which might
have been mistaken for a barrier-reef, has already been attempted to be
explained, when giving the proofs of the recent elevation of this island.--
In Cook's charts the little outlying island also of EOAIGEE, is represented
as fringed; coloured red.--EOUA.  I cannot make out from Captain Cook's
charts and descriptions, that this island has any reef, although the bottom
of the neighbouring sea seems to be corally, and the island itself is
formed of coral-rock.  Forster, however, distinctly ("Observations," page
14) classes it with high islands having reefs, but it certainly is not
encircled by a barrier-reef and the younger Forster ("Voyage," volume i.,
page 426) says, that "a bed of coral-rocks surrounded the coast towards the
landing-place."  I have therefore classed it with the fringed islands and
coloured it red.  The several islands lying N.W. of Tongatabou, namely
ANAMOUKA, KOMANGO, KOTOU, LEFOUGA, FOA, etc., are seen in Captain Cook's
chart to be fringed by reefs, in several of them are connected together.
From the various statements in the first volume of Cook's "Third Voyage,"
and especially in the fourth and sixth chapters, it appears that these
reefs are of coral-formation, and certainly do not belong to the barrier
class; coloured red.--TOUFOA AND KAO, forming the western part of the
group, according to Forster have no reefs; the former is an active
volcano.--VAVAO.  There is a chart of this singularly formed island, by
Espinoza: according to Mr. Williams it consists of coral-rock: the
Chevalier Dillon informs me that it is not fringed; not coloured.  Nor are
the islands of LATTE and AMARGURA, for I have not seen plans on a large
scale of them, and do not know whether they are fringed.

NIOUHA, 16 deg S., 174 deg W., or KEPPEL Island of Wallis, or COCOS Island.
From a view and chart of this island given in Wallis's "Voyage" (4to
edition) it is evidently encircled by a reef; coloured blue: it is however
remarkable that BOSCAWEN Island, immediately adjoining, has no reef of any
kind; uncoloured.

WALLIS Island, 13 deg S., 176 deg W., a chart and view of this island in
Wallis's "Voyage" (4to edition) shows that it is encircled.  A view of it
in the "Naut. Mag." July 1833, page 376, shows the same fact; blue.

ALLOUFATOU, or HORN Island, ONOUAFU, or PROBY Island, and HUNTER Islands,
lie between the Navigator and Fidji groups.  I can find no distinct
accounts of them.