FORMA TION F " ATOLLST 389 



Take the instance of an island slowly sinking. 

 The coral forms at first a continuous reef around the 

 coast ; as the level sinks, the coral grows upward. 

 The island diminishes at the same time in size, and 

 leaves a channel between it and the reefs, which then 

 continue their growth. A time would at length come 

 wh(m a coral reef, in the form of a ring, would be 

 visible surrounding a lagoon, in the middle of which 

 would be seen the remains of the primitive island. 

 At last, when the island shall have completely dis- 

 appeareil, the coral ring — thanks to the persistent 

 efibrts of the coral insect — will remain ; it will sur- 

 round a lake of less depth in the centre than at the 

 sides. Later, the earth continuing to sink, the depth 

 of the middle of the lake will continue to increase, 

 whilst the edges will be slowly filled up with the 

 debris arising from the disintegration of a portion of 

 the coral reef. The lake will have become like any 

 other — :the atoll will be complete ; it will itself ])er- 

 haps finally disappear, or increase in size, if the 

 movement of the earth's crust should change its 

 direction. 



The same theory evidently explains the forma 

 tion of coast reefs and barrier reefs, since these are 

 nothing more than the elements of the cUoll built in 

 a peculiar situation relative to the land.. 



