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CORAL REEFS.

MR. DARWIN, some years ago, prepared a map in which were put down all the reefs of coral which surround the numerous islands in the Pacific. In one almost straight line of ten thousand miles, from Pitcairn Island to China, those reefs stud the vast ocean. The reefs are classified, for convenience, into three groups: 1. The atoll is a circular or curved ridge of coral, visible at low-water, but nearly covered at high water, and having a tranquil lake in the centre. The diameter varies from one mile to sixty miles, in different examples; and the shape is usually an irregular oval. There is generally a profound depth of ocean at a very short distance from the atoll. In one case, the depth is a thousand feet at a distance of less than a quarter of a mile; but, far more noteworthy than this, there is one atoll at two hundred feet from which no soundings could be found with twelve hundred feet of line; and another where seven thousand feet found no bottom at a mile and a quarter distance! The interior lake or lagoon is never profoundly deep. We may therefore picture to ourselves an atoll as the top of a conical submarine mountain, with a kind of crater at the summit. 2. The barrier reef differs from an atoll in having one or more islands within it; it forms, in fact, a barrier around an island or islands at some considerable distance, and separated by a moat of deep water. Some of them run along parallel to the shore; In some the islands have joined to form a continuous strip of dry land; while in many instances the island forms a very lofty mountain. 3. The shore reef resembles a barrier reef in having land within or near it; but the dry land is very near

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and the intervening water very shallow; while in most instances there are no islands or islets, the whole reef being submerged at high-water. In all the three kinds—atoll, barrier, and shore —the reef has been formed by countless myriads of coral insects, working at the construction of their hollow dwellings. Mr. Darwin, by tracing a local connection between volcanoes and reefs, arrived at a conclusion that, wherever an atoll or a barrier reef has been formed, the bed of the ocean has subsided; while at the spots where shore reefs occur, the bed of the sea is either uprising or stationary. Islands and mountains in the Pacific have been submerged by the subsidence of the ocean bed; and when the subsidence had taken place to a certain extent, coral insects set to work at their busy labors; for, whether in the Pacific or the Mediterranean, the insect always works in the water, but at no great depth below the surface. All three kinds are satisfactorily accounted for on this view, as being in three stages of development. The shore reefs are formed first, as a fringe of coral around the coast of an island; by further subsidence, each becomes in time a barrier reef; and each of the latter develops into an atoll reef, by the insects constantly building at the top of it.