Chapter 17

[41] For instance in S. America from latitude 34°, 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.

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 Dépôt général 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° N., 109° 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 _Actæon_ 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° 4′ S., and 136° 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 8), 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,”