CORAL REEFS 



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Darwin, on his return from the voyage in the Beagle, 

 first published in 1840 his well-known theory of the 

 formation of coral atolls, and their cousins the barrier 

 reefs, which, at some distance from the shore, encircle 

 an island or extend along a coast line, leaving a con- 

 siderable passage between them and the land. 



-> r B 



Fig. 1 



Suppose Fig. 1 to represent an island along whose 

 edges corals have begun to grow, as shown by the 

 shaded portion A. Then, according to Darwin's theory, 

 if we imagine the island gradually to subside at the same 

 rate at which the coral grows, we would in time find a 

 condition shown in the dotted lines, where B B repre- 

 sents the new sea level. A smaller island is now sur- 

 rounded by a barrier reef, the passage between them 

 caused by the favorable position of the outer rim for a 

 more luxuriant growth of coral. 



Fig. 2 



Let Fig. 2 represent the second condition of the is- 

 land, the shaded parts showing the coral growth. Now 



