346 PHYSIOGRAPHY 



bonate) live chiefly in shallow water, and the seas in which the 

 limestones of the land were formed were generally, if not always, 

 shallow. 



Coral reefs. Coral reefs are of so much interest and importance 

 that they deserve a special word. The little animals (called 

 polyps) which secrete the coral live (1) where the water is 120 feet 

 or less in depth, (2) where the temperature never falls below about 

 68 F., (3) where the water has the salt ness of normal sea-water, 

 (4) where the water is nearly free from sediment, and (5) where it 

 is subject to some movement by the wind. Where these conditions 

 exist, polyps thrive and make reefs, and the reefs may become 

 islands. Polyps flourish about many tropical islands, and along 

 some* continental coasts, as along the eastern coast of Australia. 

 They also flourish in some places far from islands or continents, 

 where there is shallow water of the right temperature. 



Fig. 271. Diagram of a fringing reef. Fig. 272. Diagram of a barrier reef. 



Figs. 271 and 272 show coral reefs. Those which are far enough 

 from the land to leave a somewhat wide and deep belt of water 

 (lagoon) inside, are barrier reefs; those close to the land are fringing 

 reefs. It seems probable that fringing reefs sometimes become 

 barrier reefs by the sinking of the island or coast where they occur. 

 Where this is the case, the sinking should not go on faster than 

 the polyps build up the reef, that is, but a few inches a century. 

 Barrier reefs arise in other ways also. Coral reefs are usually 

 interrupted where fresh water descends from the land, so that a 

 reef rarely surrounds an island completely, and is rarely continuous 

 for great stretches along any coast. 



A barrier reef about a small island may become an island or 

 atoll (Fig. 273) by subsidence. Coral islands may also arise by the 

 growth of reefs on volcanic cones or other islands which do not 

 rise to the surface of the water. 



