Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/399

1869.] RATTRAY CAPE-YORK PENINSULA. 303 probably be found in the shoaling of the inner barrier-reef route and the numerous openings through it, as well as the reef itself, which will one day be out of water, like Raine and other islands, when the region is resurveyed many years hence.

The great barrier-reef of Australia is evidently formed by the growth of coral on the summit of a lengthy submarine hilly ridge, which runs at a distance of from fifty miles, at its southern extremity, midway down the east coast (lat. 22° S.), to ten miles, near Cape York, leaving a long shallow valley between it and the mainland, now converted into the navigable passage named the inner barrier- reef route, the depth of which varies only from 10 to 50 fathoms; whereas the outer aspect of the reef shelves steeply like a wall, no bottom being found only a short distance off at 100 to 250 fathoms, where the clear and dark-green waters and long ocean-swell form a marked contrast to the light green of the tranquil but turbid inner passage. The reef may be said to consist of the reef proper and islands. The former, which constitutes the main mass of this lengthy coral tract, consists of a series, of broad flat irregular more or less rounded or oval ledges of coral in full activity, covering the tops of the submarine hills, occasionally dry but usually close to the surface at low water, and sometimes many miles long, their position at high water being marked by a line of surf or the light green hue of the shallow water over them. On the other hand the islands are of three different kinds, viz. igneous, coral islands, and sand-banks.

1st. Occasionally the higher peaks of this barrier ridge project well out of the water, as in the Forbes, Hardy, Cockburn, Murray, and other islands, which are in a line with and really form part of the range — and Sunday, Lizard, Eagle, the Franklands, Dunk, and others, which lie between it and the coast. Round these projecting masses of crystalline rock we find the usual fringing reef of coral, of species identical with those growing on the summits of the ridge which forms the barrier proper. But on their sides we find no evidence of coral growth, proving that though they must have, at least, once been submerged, like the adjacent continent, this must have been when no coral grew in the Australian seas.

2nd. Here and there, however, we meet with islands of another class, of which Raine, Cairncross, the Howick group, &c. may be taken as types, a few of which, like the first named, form part of the reef, but the majority, like the last of the three, are met with in clusters, between it and the mainland. Raine island, taken as an example, which possesses its own special and active fringing reef, is low, flat, about one-third of a mile long and a quarter of a mile broad; it rises about 10 feet above high water, and consists of hard compact brecciated coral conglomerate, with a shelving beach of coarse coralline and shelly sand, and a scanty superstratum composed of the coral debris sparingly mixed with vegetable matter, and a thin layer of guano deposited by the numerous turtle and flocks of terns, gannets, and other aquatic birds that, like the former, make this their headquarters and favourite breeding-place. The whole constitutes a soil capable