ABSTRACT
T 0 the west of the Asiatic mainland, of the archipelago of Indonesia and of the continent of Australia there extends the vast ocean of the Pacific, with a span of over eight thousand miles from Southern China to Mexico. Eut the Pacific is not an empty ocean; in its western and central parts island groups are more closely studded than in any other great ocean of the world. These islands are not scattered aimlessly, but are the pinnacles and crowns of submerged mountain arcs, the great majority of which, both exposed and submarine, have been formed by volcanic action. Thus the islands often stand out boldly from the sea, rising steeply beyond a narrow foreshore to heights of several thousand feet. Elsewhere thick deposits of sea muds have been raised above the surface to form lower and smoother islands of stratified rocks. On the tops of the submerged mountain peaks, however, another process of land building has been slowly going on-the formation of limestone rock by the building of corals. Coral polyps are marine insects which construct, each for itself, a hard casing of lime which endures long after it is dead. Building continually out and over their dead ancestors, the polyps produce great thicknesses of coral which are in time worn down by current and wave into a consolidated limestone. But these little animxls can only live and build within a narrow range of physical conditions. For coral growth the water must be warm, shallow, salt and clear. Thus the coral reefs of the world are restricted to tropical seas in which the mean surface temperature of the water is about 70" F. The polyps cannot live a t depths below about thirty fathoms and are killed off where the entry of a river freshens the sea water and clouds it with mud. The multitude of small islands and shallow submarine ridges in clear warm water afford ideal conditions for coral building in the 'South Seas' of the Pacific. The formations are of three main kinds: fringirtg reefs, which lie close along the
I74 HABITAT, ECONOMY AND SOCIETY shore; barrier reefs, which stand out perhaps a half to several miles from the coast-line on the far side of a lagoon of calm deep water; and atolls, reefs which surround a central lagoon in which there is no island. The coral of the barrier reefs and atolls extends to far greater depths than the polyps are now able to build, and it is clear that they have been built up by progressive sinking, relative to the level of the sea, of the coasts and islands which they fringe. Finally, considerable thicknesses of coral have on some islands been lifted above the sea-level, forming low plateaux and coastal plains. With the problems of the nature and causes of these movements we are not here concerned, but it is essential to understand the general character of the coral formations, for they are the background of so much of Oceanian life.
