Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

556 CORAL FORMATIONS. April, 1836. in New Caledonia, where the reefs extend no less than 140 miles beyond the island. The great Barrier which fronts the N.E. coast of Australia, forms a third class of reef. It is described by Flinders as. having a length of nearly one thousand miles, and as running parallel to the shore, at a distance of between twenty and thrty miles from it, and, in some parts, even of fifty and seventy. The great arm of the sea thus included, has a usual depth of between ten and twenty fathoms, but this increases towards one end to forty and even sixty. This probably is both the grandest and most extraordinary reef now existing in any part of the world. It must be observed, that the reef itself in the three classes, namely, lagoon, encircling, and barrier, agrees in structure, even in the most minute details : but these I have not space here even to allude to. The difference entirely lies in the absence or presence of neighbouring land, and the relative position which the reefs bear to it. In the two last- mentioned classes, there is one difficulty in undertaking their origin, which must be pointed out. Since the time of Dam- pier it has been remarked, that high land and deep seas go together. Now when we see a number of mountainous islands coming abruptly down to the sea- shore, we must suppose the strata of which they are composed, are con- tinued with nearly the same inclination beneath the water. But, in such cases, where the reef is distant several miles from the coast, it will be evident upon a little consi- deration, that a line drawn perpendicularly from its outer edge down to the solid rock on which the reef must be based, very far exceeds that small limit at which the efficient lamelliform corals exist. In some parts of the sea, as we shall hereafter mention, reefs do occur which fringe rather than encircle islands — the distance from the shore being so small, where the inclination of the land is great, that there is no difficulty in understand- the growth of the coral. Even in these " fringing" reefs, as I shall call them in contradistinction to the " encircling," the