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 CLASSES 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.