Page:Popular Science Monthly Volume 1.djvu/280

268 states that the astræas occur at the bottom of the reef. Next in order, the brain-coral, porites, madrepores, and on the surface the light, branching varieties which are the shrubbery of the coral world. This arrangement may occur where the bottom on which the reef stands is immovable, or has remained without change of level since the reef commenced. But how it could occur in a coral mass 2,000 feet thick, growth being limited to a depth of 120 feet, is not entirely obvious. In order to explain the enormous depth of coral-reefs upon the submerged lands of the Pacific, it is necessary to consider the further well established fact, first suggested by Charles Darwin, that the lands and ocean bed have gradually subsided. The subsidence has been often at the same slow rate that coral-reefs have increased by upward growth. It seems inevitable from this that the builders are at work on the upper portions of the reef; certainly it is here only that the work of elevation can go on. Summing up this subject, Prof. Dana says: "Reef-building corals of the different groups grow together promiscuously at different depths up to low-tide level. The largest astræas, mændrines (brain-coral), porites, and other kinds, have been seen by the author, constituting the upper part of the growing reef." The coral polype flourishes only in the belt of warm waters which lies in and near the tropics. A temperature lower than 68° Fahrenheit is fatal to them. The great reefs abound and grow with greatest vigor in the zone of greatest heat.

Surrounding most of the tropical islands are two principal reefs, one fringing the shore; the other, called the barrier-reef, lying seaward, sometimes more than 15 miles from the land. The intervening space is often filled with minor reefs and a gorgeous wealth of coral vegetation.

Here lie immense platform-reefs, a shell of coral covering the bottom beneath the shallow waters. These together make up the coral-reef ground of the island. West of the two larger Feejee Islands are 3,000 square miles of reef-ground. New Caledonia has a reef along its western shores a distance of 250 miles. The great Australian barrier, lying east and northeast of that island, forms a broken reef, 250 miles in length. On these outer reefs the waves forever break, and here, where the plunge of the surf is most furious and persistent, the polypes flourish with greatest vigor, and open their many-colored petals to the life-giving waters, as do thirsty flowers to the welcome rain. On every dead space delicate moss-like and lichen-like corals quickly form their thin, hard crusts.

Outside the reefs there occurs, in many places, a coral growth alike curious and interesting. In isolated patches are found immense mushroom-shaped masses called coral-heads. One is described by Whipple, cited by Dana, standing in water 50 feet deep, near Turk's Island. Its trunk is about 15 feet in diameter, supporting a tabular mass 100 feet in diameter, the top being bare at low tide. When these corals reach the surface, growth in that direction ceases, but may