[11] “Memoires pour servir à une description Geolog. de France,” tome iv, p. 216.
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[[12]] 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.
[12] Beechey’s “Voyage,” 4to ed., p. 44.
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.[[13]] Although Chamisso does not state that the masses of Nulliporæ form points or a mound, higher than the flat, yet I believe that this is the case; for Kotzebue,[[14]] 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,[[15]] Whether a smooth convex mound of Nulliporæ, 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.
[13] Kotzebue’s “First Voyage,” vol. iii, p. 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 Nulliporæ. 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 Nulliporæ are closely related to those described on the coral-reefs, but I believe are of different species.
[14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in his excellent memoir in the Geological Transactions (vol. ii, p. 105), alludes to the rocky points mentioned by Kotzebue, and infers that they consist of Serpulæ, 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 Serpulæ hold the same position, relatively to the action of the sea, with the Nulliporæ on the coral-reefs in the Indian and Pacific Oceans.
[15] Captain Moresby, in his valuable paper “on the Northern atolls of Maldivas” (Geographical Journal, vol. v), says that the edges of the reefs there stand above water at low spring-tides.
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[[16]] 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,[[17]] 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.
[16] Kotzebue’s “First Voyage,” vol. iii, p. 144.
[17] See also Moresby on the Northern atolls of the Maldivas, Geographical Journal, vol v, p. 400.