Coral Reefs

Page 56

The inhabitants of the Maldiva Archipelago, as long ago as 1605, declared, "that the high tides and violent currents were diminishing the number of the islands" (See an extract from Pyrard's Voyage in Captain Owen's paper on the Maldiva Archipelago, in the "Geographical Journal", volume ii., page 84.): and I have already shown, on the authority of Captain Moresby, that the work of destruction is still in progress; but that on the other hand the first formation of some islets is known to the present inhabitants. In such cases, it would be exceedingly difficult to detect a gradual subsidence of the foundation, on which these mutable structures rest.

Some of the archipelagoes of low coral-islands are subject to earthquakes: Captain Moresby informs me that they are frequent, though not very strong, in the Chagos group, which occupies a very central position in the Indian Ocean, and is far from any land not of coral formation. One of the islands in this group was formerly covered by a bed of mould, which, after an earthquake, disappeared, and was believed by the residents to have been washed by the rain through the broken masses of underlying rock; the island was thus rendered unproductive. Chamisso (See Chamisso, in Kotzebue's "First Voyage," volume iii., pages 182 and 136.) states, that earthquakes are felt in the Marshall atolls, which are far from any high land, and likewise in the islands of the Caroline Archipelago. On one of the latter, namely Oulleay atoll, Admiral Lutke, as he had the kindness to inform me, observed several straight fissures about a foot in width, running for some hundred yards obliquely across the whole width of the reef. Fissures indicate a stretching of the earth's crust, and, therefore, probably changes in its level; but these coral-islands, which have been shaken and fissured, certainly have not been elevated, and, therefore, probably they have subsided. In the chapter on Keeling atoll, I attempted to show by direct evidence, that the island underwent a movement of subsidence, during the earthquakes lately felt there.

The facts stand thus;--there are many large tracts of ocean, without any high land, interspersed with reefs and islets, formed by the growth of those kinds of corals, which cannot live at great depths; and the existence of these reefs and low islets, in such numbers and at such distant points, is quite inexplicable, excepting on the theory, that the bases on which the reefs first became attached, slowly and successively sank beneath the level of the sea, whilst the corals continued to grow upwards. No positive facts are opposed to this view, and some general considerations render it probable. There is evidence of change in form, whether or not from subsidence, on some of these coral-islands; and there is evidence of subterranean disturbances beneath them. Will then the theory, to which we have thus been led, solve the curious problem,--what has given to each class of reef its peculiar form?

(PLATE: WOODCUT NO. 4.

AA--Outer edge of the reef at the level of the sea.

BB--Shores of the island.

A'A'--Outer edge of the reef, after its upward growth during a period of subsidence.

CC--The lagoon-channel between the reef and the shores of the now encircled land.

B'B'--The shores of the encircled island.

N.B.--In this, and the following woodcut, the subsidence of the land could only be represented by an apparent rise in the level of the sea.

PLATE: WOODCUT NO. 5.

A'A'--Outer edges of the barrier-reef at the level of the sea. The cocoa-nut trees represent coral-islets formed on the reef.

CC--The lagoon-channel.

B'B'--The shores of the island, generally formed of low alluvial land and of coral detritus from the lagoon-channel.

A"A"--The outer edges of the reef now forming an atoll.

C'--The lagoon of the newly formed atoll. According to the scale, the depth of the lagoon and of the lagoon-channel is exaggerated.)

Let us in imagination place within one of the subsiding areas, an island surrounded by a "fringing-reef,"--that kind, which alone offers no difficulty in the explanation of its origin.

Coral Reefs Page 57

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Charles Darwin

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