inclosed shells or other animal remains; in other places, it is made up of broken and comminuted corals cemented together, or of masses of coral standing as they slowly grew, with the spaces between the separate clumps filled up with coral sand and triturated fragments and grit of coral and shell. The thickness of the reefs is often very great, reaching in many cases to thousands of feet. At the Fijis, the reef can hardly be less than 2000 or 3000 feet thick. Below a depth of one hundred feet, all the coral rock is dead, and since the coral zoophytes do not live at greater depths than this, it follows that the bed of the sea in which coral reefs occur must have slowly subsided during a long course of ages. Mr Darwin was the first to give a reasonable explanation of the origin of coral reefs. Briefly stated, his explanation is as follows: The corals began to grow first in water not exceeding one hundred feet in depth, and built up to the surface of the sea, thus forming a fringing reef at no great distance from the land. This initial step is shewn at A, B, in the accompanying section across a coral island. A, A, are the outer edges of the fringing reef; B, B, the shores of the island; and S1 the level of the sea. Subsidence ensuing, the island and the sea-bottom sink slowly down, while the coral animals continue to grow to the surface--the building of the reef keeping pace with the subsidence. By and by the island sinks to the level S2, when B´, B´, represent the shores of the now diminished island, and A´, A´, the outer edges of the reef, which has become a barrier reef; C, C, being the lagoon between the reef and the central island. We have now only to suppose a continuance of the submergence to the level S3, when the island disappears, its site being occupied by a lagoon, C´--the reef, which has at the same time become an atoll, being shewn at A´´, A´´.