REEF or Shoal. Sha'b, ARAD. Darwin (Re searches, p. 555) describes the lagoon islands, the encircling reefs, and the barrier reefs. Coml islands are arranged by Darwin into the atoll or lagoon island, a coral margin with a lagoon in the centre ; barrier reefs, stretching along a vast extent of coast ; and encircling coral reefs, which are merely fringes of coral along the margin of a shore. Von Birch is of opinion that the lagoon island is the margin of a sub marine cmter on which the coral animal luta built its wonderful structure. The barrier reefs, according to Darwin, are due to sub sidence. Itt a sheltereil archipelago, they rarely rise to the surface. But in an open ocean, rolling waves and breakers throw up a barrier of broken coml far above the usual high water-mark.
In New Caledonia the encircling reef extentli 140 miles beyond the island. At Vanikoro, the reef runs two or three miles from the shore, from which it is separated by a channel from 30 to 50 fathoms deep. Externally, the reef rises from an ocean profoundly deep.
The great barrier reef which fronts the N.E. coast of Australia for nearly 1062 mile,s, runs paral lel to the shore, at distances ranging from 20 to 70 miles, the enclosed sea, 31,860 square nines in area, varying in depth from ten to sixty fathoms. Sir Charles Lyell and Darwin think (557) that the great depths of the marginal seas its caused by the subsidence of the land, the comls raising their structure as the lands subside.
There are innumerable coral reefs and coral islands, but Darwin has satisfactorily shown that atolls, or annular reefs, were originally fringing reefs constructed around islands that have since subsided. Coral reefs have thus been
divided into three classes, according to their geological character,—the shore reefs fringe the shores of continents or islands ; the encircling reefs or barrier reefs ; the third, enclosing a lagoon, is called an atoll, or lagoon island, and is a ring or annular breakwater around an interior lake. In the Archipelago and the Pacific are many coral islands or atolls. An atoll differs from an encircling barrier reef only in the absence of land within its central expanse ; and a barrier reef differs from a fringing reef, in being placed at a much greater distance from the land, with reference to the probable inclination of its sub marine foundation, and in the presence of a deep water lagoon-like space or moat within the reef. Atolls sometimes constitute a great circular chain enclosing a deep basin, but openin,g by ono or more deep breaches into the sea. Sometimes they surround a little island by a girdle of reefs • or form the immediate edging or border of an island or continent. Atolls occur in the Pacific, in the Chinese Seas, in the Marianne and Philippine Islands, Maldives, Lacrndives, and Sunda group. —Darwin on the Structure and Distribution of Coral Reefs; Hartwig.