LAGOON ISLAND

In Physical Geography, the name given to a particular kind of coral island. The Atoll consists of a chaplet or ring of coral, enclosing a lagoon or portion of the ocean in its centre. The average breadth of the part of the ring which rises above the surface of the sea is nearly a quarter of a mile, and it seldom rises higher than from six to ten or twelve feet above the surface. On the outer side the ring gradually shelves down to the distance of 100 or 200 yards from its edge, so that the sea deepens to about 25 fathoms, beyond which the sides plunge almost perpendicularly into the unfathomable depths of the ocean. On the lagoon side, where the water is calm, the bounding ring or reef shelves into it by a succe ion of ledges, also of living coral, but of a different species from those which build the exterior wall, and the foundation of the whole ring. The size varies from two to ninety miles in diameter. The lagoon islands are widely I spread, but chiefly in the Pacific Ocean. Most frequently I they occur in elongated archipelagos, extending in ' groups over a large surface; the Carolina Islands, for I example, extend to 1000 miles. Sometimes the lagoons have islands within them: Otaheite, the largest of the I Society group, is a remarkable instance of this formation. The lagoon which encompa es it is thirty fathoms I deep, and hemmed in fiom the ocean by a coral band of ' the usual kind, at a distance varying from half a mile to I three miles. {Somerville's Physical Geography.) j [s. 1369]