Page:Popular Science Monthly Volume 85.djvu/218

214 have cut three ravines down the slopes, and in former times, before the coral reef grew seaward to protect the island, the sea commenced to cut into the shores, forming a precipice about 20 feet high along the southeastern side of the island.

But, after the volcanic fires had ceased, corals began to grow along the shores and soon the island was surrounded by a fringing reef. The steep seaward slope and outer edges of this reef provided the best foothold for the growth of corals, and no sooner did the old ones die than new growths took possession of the coveted space and thus the reefs pushed seaward from the shores and the old volcano was protected from the attack of the breakers which now break impotently upon the outer edge of the ever-widening reef.

As is always the case under these conditions, the reef advanced seaward most rapidly against the wind, for corals thrive best in agitated water which is free from silt. Thus the reefs are three times as wide on the southeastern as on the northwestern sides of the island, for not only is the water too calm for the most luxuriant coral growth on the leeward side, but the mud which is washed over the reef-flats during the rainy season interferes seriously with coral growth. It is due also to this fact that the reef—or platform—is narrow wherever the silt from the streams washes over it, and wide wherever it is covered with pure ocean water and exposed to the full force of the breakers.

Maër Island is oval, about three miles long and one and a half wide and the southeast trade-wind causes the water currents to sheer in opposite directions from the middle of the southeast shore, and eddies are formed at both ends of the island, and these deposit water-washed sand along the leeward side, thus forming dunes at the ends and a sand beach along the middle of the leeward side, and converting the originally oval shape of the island into a crescent with its horns and concavity directed down the wind. Indeed Hedley and Griffith-Taylor, and Wood-Jones have shown that the crescentic shapes of coral atolls are formed in this manner in obedience to the direction of the prevailing winds and currents.

In short, the width and character of the reefs surrounding Maër Island are determined by the shore conditions; and it is quite clear that the island has not developed in the midst of a preexisting reef-flat, but that the volcano was formed before the modern reefs began to grow around it. Moreover, they are all fringing reefs that have grown outward from the shore and thus the "lagoon" of the great southeast reef is only 18 inches deep and its bottom is of hard coral with none of the mud and occasional reef patches seen in the bottoms of all lagoons between barrier reefs and the shore.

Indeed according to a theory which has been put forth by Penck, and more recently by Professor R. A. Daly, barrier reefs originated