FORMATION OF CORAL REEFS. 153 



be situated in that area of subsidence, it will, 

 of course, sink with the floor on which it rests, 

 carrying down also the Coral wall to a greater 

 depth in the sea. In such instances, if the rate 



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of subsidence be more rapid than the rate of 

 growth in the Corals, the island and the wall 

 itself will disappear beneath the ocean. But 

 whenever, on the contrary, the rate of increase 

 in the wall is greater than that of subsidence in 

 the island, while the latter gradually sinks below 

 the surface, the former rises in proportion, and 

 by the time it has completed its growth the cen- 

 tral island has vanished, and there remains only 

 a ring of Coral Reef, with here and there a 

 break perhaps, at some spot where the more 

 prosperous growth of the Corals has been checked. 



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If, however, as sometimes happens, there is no 

 such break, and the wall is perfectly uninter- 

 rupted, the sheet of sea-water so enclosed may 

 be changed to fresh water by the rains that are 

 poured into it. Such a water-basin will remain 



7* 



