and commence to grow. The reef rises, but, as before remarked, corals grow most rapidly in the purest waters, and thus it is that reefs often seem to crowd against the waves without and assume a wall-like aspect. But, owing to the wash of the land within, and the discharge of streams in some instances, the polypes are less healthy and their growth more precarious. These causes modify the form of the reef. But, however modified, the reef fringes or encircles the land (Fig. 13). We have already remarked that coral-reefs can attain no greater thickness than about 120 feet, unless there occurs with their growth a simultaneous subsidence of the land on which they rest; but, with that coincidence, the growth of the reef may continue so long as the subsidence goes on.
It is obvious that, with this sinking of the land, the area of the island must diminish, the sea and its accompanying corals gradually encroaching upon its shores. At last the land disappears. Then we have a lake or lagoon over its former site, surrounded by coral-reefs—for the builders have not been at rest. All the features of coral growth continue, but the land with its wealth of vegetation is buried. A coral floor has formed over it. This is the history of hundreds, perhaps thousands of former islands in the Pacific alone; and the great reefs, from which the surf sends up an incessant wail, are the monuments of this ocean-cemetery.
Fig. 13.
High Island, with Barrier and Fringing Beef.
The encircling reefs, with the lagoon, are called an atoll, which is only another name for a coral-island. This effect is well shown in Fig. 14, page 271.
Moreover, the force of waves and lifting-power of water have broken fragments of coral and coral-rocks, and thrown them upon the reef. The mass may have been already weakened by the perforations of in-numerable boring worms and mollusks which burrow in the reefs. Thus in places beaches have been formed 10 or 12 feet in elevation above the ocean. They are composed of coral sand made fine and drifted by waves and winds, fragments of shells, bones of fishes, and other matters drifted thither by the sea.
The beauty of the completed atoll must be given in Prof. Dana's words: "When first seen from the deck of a vessel, only a series of dark points is descried just above the horizon. Shortly after, the points
