Chapter VI: On the Distribution of Coral-Reefs with Reference to the (1)

THEORY OF THEIR FORMATION

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):[1] 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.

[1] Inasmuch as the coloured map would have proved too costly to be given in this series, the indications of colour have been replaced by numbers referring to the dotted groups of reefs, etc.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.

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.[2]

[2] 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_ (“Geol. Trans.,” vol. v, p. 601). I have added a volcano to Albemarle Island, one of the Galapagos Archipelago (the author’s “Journal of Researches,” p. 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,” p. 29) that Toofoa and Proby Islands are active volcanoes. I infer from Hamilton’s “Voyage in the _Pandora_” (p. 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, p. 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, vol. xii, p. 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,” vol. ii, p. 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 (p. 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.” vol. iv, p. 458), says there are two active volcanoes in latitude 23° 30′, and in latitude 24°: but I have not coloured them. From the statements in Beechey’s “Voyage” (p. 518, 4to edit.) 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, p. 832), that there are two active volcanoes on the eastern side of Formosa. In Torres Straits, on Cap Island (9° 48′ S., 142° 39′ E.) a volcano was seen burning with great violence in 1793 by Captain Bampton (see Introduction to Flinders’ “Voyage,” p. 41). Mr. M’Clelland (Report of Committee for investigating Coal in India, p. 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_, vol. xxxviii, p. 385). In Berghaus’ “Phys. 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,” p. 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, p. 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[3] mentions many atoll-formed and encircling coral-reefs, some of which lie within the barrier, and others may be said (for instance between lat. 16° and 13°) to form part of it. Flinders[4] has described an atoll-formed reef in lat. 10°, 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.

[3] Sailing directions, appended to vol. ii of his “Surveying Voyage to Australia.”

[4] “Voyage to Terra Australis,” vol. ii, p. 336.

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,[5] 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.[6]

[5] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier in his “Report on the Voyage of the ‘Astrolabe’” (p. cxi, vol. 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, Fig. 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 (vol. ix, p. 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.

[6] Mr. Couthouy states (“Remarks,” p. 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 Tridacnæ 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° N. lat.) 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, p. 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[7] 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.

[7] “Annales des Sciences Nat.” tom. vi, p. 279, etc.

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[8] 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[9] 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.

[8] “Zoology of Captain Beechey’s Voyage,” p. 176. See also MM. Quoy and Gaimard in “Annales de Scien. Nat.” tom. vi.

[9] “Remarks on Coral Formations,” p. 51.

In the southern part of the Low Archipelago, Elizabeth Island is described by Captain Beechey,[10] 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[11] 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.

[10] Beechey’s “Voyage in the Pacific,” p. 46, 4to ed.

[11] Couthouy’s “Remarks,” p. 34.

_Savage Island_ (S.E. of the Friendly group), is about forty feet in height. Forster[12] describes the plants as already growing out of the dead, but still upright and spreading trees of coral; and the younger Forster[13] 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.”[14] 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[15] 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[16] 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.

[12] “Observations made during Voyage round the World,” p. 147.

[13] “Voyage,” vol. ii, p. 163.

[14] Cook’s “Third Voyage” (4to ed.), vol. i, p. 314.

[15] _Ibid_., vol. i, p. 235.

[16] “Remarks on Coral-Formations,” p. 50.

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[17] 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,[18] and Chamisso,[19] 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.[20]

[17] “Voyage de la _Coquille_,” Part. Zoolog.

[18] Freycinet’s “Voyage autour du Monde.” See also the “Hydrographical Memoir,” p. 215.

[19] Kotzebue’s “First Voyage.”

[20] Lutké’s “Voyage,” vol. ii, p. 304.

In the _East Indian Archipelago_, many authors have recorded proofs of recent elevation. M. Lesson[21] 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[22] 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,”[23] 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[24] 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[25] (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[26] 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.

[21] Partie Zoolog., “Voyage de la _Coquille_.”

[22] “Ann. des Scien. Nat.” tom. vi, p. 281.

[23] Translated by Windsor Earl, chapters vi, vii.

[24] “Geolog. Transact.” 2nd series, vol. i, p. 403. On the Peninsula of Malacca, in front of Pinang, 5° 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.” vol. xviii, part ii, p. 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, p. 131) mounds of shells lying two miles inland on this line of coast.

[25] “Notices of the East Indian Arch.” Singapore, 1828, p. 6, and Append., p. 43.

[26] Captain B. Hall, “Voyage to Loo Choo,” Append., pp. xxi and xxv. Captain Beechey’s “Voyage,” p. 496.

Dr. Davy[27] describes the northern province of _Ceylon_ 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.[28] _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[29] 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° 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.

[27] “Travels in Ceylon,” p. 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.

[28] Dr. Benza, in his “Journey through the N. Circars” (the _ Madras Lit. and Scient. Journ._ vol. 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.

[29] Owen’s “Africa,” vol. ii, p. 37, for Madagascar; and for S. Africa, vol. i, pp. 412 and 426. Lieutenant Boteler’s narrative contains fuller particulars regarding the coral-rock, vol. i, p. 174, and vol. ii, pp. 41 and 54. See also Ruschenberger’s “Voyage round the World,” vol. i, p. 60.

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. Rüppell[30] 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°; but that south of 26°, 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 (lat. 15° 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. Rüppell says that the modern beds attain a height of only thirty to forty feet, Mr. Burton[31] 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.

[30] Rüppell, “Reise in Abyssinien,” Band i., s. 141.

[31] Lyell’s “Principles of Geology,” 5th ed., vol. iv, p. 25.

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[32] 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.[33]

[32] Ainsworth’s “Assyria and Babylon,” p. 217.

[33] On Florida and the north shores of the Gulf of Mexico, Rogers’ “Report to Brit. Assoc.” vol. iii, p. 14.—On the shores of Mexico, Humboldt, “Polit. Essay on New Spain,” vol. i, p. 62. (I have also some corroborative facts with respect to the shores of Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th ed., vol. iv, p. 22.—Santa Cruz and Barbadoes, Prof. Hovey, “Silliman’s Journal”, vol. xxxv, p. 74.—St. Domingo, Courrojolles, “Journ de Phys.” tom. liv., p. 106.—Bahamas, “United Service Journal”, No. lxxi, pp. 218 and 224. Jamaica, De la Beche, “Geol. Man.” p. 142.—Cuba, Taylor in “Lond. and Edin. Mag.” vol. xi, p. 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[34] 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,[35] 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.

[34] Ellis, in his “Polynesian Researches,” was the first to call attention to these remains (vol. i, p. 38), and the tradition of the natives concerning them. See also Williams, “Nar. of Missionary Enterprise,” p. 21; also Tyerman and G. Bennett, “Journal of Voyage,” vol. i, p. 213; also Mr. Couthouy’s “Remarks,” p. 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, p. 54. There is a passage in Von Zach, “Corres. Astronom.” vol. x, p. 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.