Page:A Treatise on Geology, volume 1.djvu/349

 CHAP. VI. yards in breadth, with the depth varying from 3 to 15 fathoms.

The dark part is calm water round the islands and within the coral reef. The arrows show the entrances and exits for vessels.

The form of the coral islands must very materially depend upon that of the base on which they happen to be built; hence their circular, lunulate, oval, or irregular forms give information as to the shape and even nature of the subjacent rocks. In most cases, the base of the small islands appears to be a volcanic crater, entire or broken; islands of volcanic rock, as Tahiti, are surrounded by rings of coral. The elevation of the coral islands is not owing to the mere accumulation by the rough action of the sea, but to a gradual rising of the low islands, and a violent subterranean movement of the lofty ones, like Tahiti, which bears on the apex of one of the highest mountains a distinct and regular stratum of semi-fossil coral, and near it, but on a lower level, a volcanic crater with two lateral gorges.

In this case, had the upward movement been gradual, why should not the coral growths have covered the edges of the crater, or rested on other points?

Mr. Darwin has recently been conducted, by a consideration of the structure of coral islands, annular and linear, whether immediately investing, or at a