Coral Reefs, or CORAL ISLANDS

Narrow islands composed of coral rock, that lie parallel with the shore of continents or islands. When small and close to shore the reef is a fringing reef; when further out and rising out of deep water, it is a barrier-reef. Encircling reefs surround islands; when the central island is sunk below the water's surface, it is an atoll. Corals flourish only where the water is le than 30 fathoms and the mean winter temperature not below 66° F.; hence IMG:content-0974.png:[graphic][merged small] these islands are usual only within 1,800 m. of the Equator. Barrier reefs can be accounted for only by supposing the ocean bottom to have sunk as the corals built upward. upward. The corals die when the surface is reached, and growth continues only on the sides of the reef. Then the breakers grind up the coral and pile the sand into ridges; aided by the wind, these may attain considerable heights. The sand becomes gradually cemented together to form coral rock.