Page:Popular Science Monthly Volume 85.djvu/219

Rh in a manner quite different from that of the recent fringing reefs of the Murray Islands. According to these students water was abstracted from the sea to form the great polar ice caps of the glacial epoch, and Professor R. S. Woodward has demonstrated mathematically that a still further lowering of level in tropical seas must have resulted from the attraction of the ice caps for the ocean surrounding them.

Thus the level of the tropical oceans may have been about 120 feet lower than at present. Now under these conditions the oceans would wash away the shores forming platforms along the tropical coasts at a level which would be about 120 feet below the present surface of the water.

Then when the ocean began again to rise, after the close of the glacial epoch, corals would grow along the outer edges of these platforms and thus Atolls and Barrier Reefs have been formed; the Atolls growing upon the truncated summits of mountains.

Andrews called attention to the fact that the platform upon the outer edge of which the Great Barrier Reef has grown, extends southward far beyond the latitude of corals, and Vaughan has observed that the platform of the Florida reef extends far northward beyond the last coral reefs. Also we may observe the Barrier Reef platform extends northward to the coast of New Guinea although the corals are killed in the wide region of the Bligh Entrance by the silt of the Fly and other great Papuan rivers. Thus it appears that the corals have merely grown as break-waters upon the seaward edges of platforms which were formed before the reefs themselves developed.

In some respects, the Pacific reefs are markedly different from those of the Atlantic. In the Pacific, one misses the beautiful sea fans and gorgonians that wave in languid grace to the rhythm of the surges in the crystal waters of Florida and the Bahamas. Instead, we find large areas of leathery-looking alcyonaria, or fleshy eight-rayed corals, Sarcophyton and Alcyonium, and in the crevices one often sees the giant clams Tridacna, their valves opened to show the beautiful mantle edges of malachite green, or blue, yellow or mottled with brown in a gamut of color, no two individuals being alike.

There are almost twice as many kinds of corals in the Pacific as in the Atlantic, and some of the more fragile of these grow luxuriantly down to depths of 60 feet or more, whereas in the Atlantic the coral reefs thrive well only in shallow water not over 20 feet in depth.

But the most striking feature which distinguishes the Pacific reefs is the development of a ridge which actually projects half a foot or more above low tide level and extends along the outer seaward edge of the reef-wall wherever the breakers dash. In the Paumotos, this ridge is dull reddish pink in color, and it is composed of a mass of stony seaweeds or nullipores of the sort called Lithothamnion, and also of