THE OCEAN 



347 



The polyps do not build the reef or the atoll above water; 

 but when they have built it up to water-level, the waves may build 

 it higher, much as they convert sand reefs into land. Once land 



Fig. 273. An atoll. (From Dana's Corals and Coral Islands, by permission of 



Dodd, Mead & Co.) 



appears, the wind may make it higher by piling up coral sand. 

 The growth of vegetation may help along the building, both by its 

 own growth and by helping the lodgment of wind-blown sediment. 





Fig. 274. Living polyps and their secretions of coral. 



Coral islands and reefs would always remain low if it were not 

 for diastrophism. There are, indeed, no very high coral islands, 

 but there are coral reefs far above sea-level along various coasts. 



