[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.