72 CORAL ISLANDS. 



In pure water of a certain temperature the deposition of 

 lime by corals is very rapid. Coral Islands, or atolls, are, as 

 a rule, nearly circular or horse-shoe shaped, the inner lagoon 

 being shallow and communicating with the open sea by a 

 channel on the leeward side. There is usually deep water 

 off the island. Coral Reefs are small and skirt the shore of 

 an island, frequently as a long ridge parallel to the shore and 

 some distance from it when they are called Fringijig Reefs, 

 or if they be large and a long way out from the shore 

 they are called Barrier Reefs, The water outside of a 

 barrier reef is often of great depth. 



Fig. 22. — Diagram to Illustrate Darwin's Theory of 

 Coral Reefs. 



Third level of sea 



with Atoll.- 



Second level of sea 

 with barrier reef. 



First level of 



^ sea with -- 

 fringing reef. 



The slow subsidence of the land causes successive changes of sea-level. 



The difficulty in accounting for the origin of coral 

 islands and coral reefs lies in the fact that the commonest 

 and best reef-builders do not find suitable conditions of 

 temperature below about 25 fathoms, whereas great depths 

 are found immediately outside atolls and barrier reefs. This 

 is overcome by various suppositions. That connected with 

 the name of Darwin assumes extensive subsidence of the 

 land, gradually converting a fringing reef round a peak 

 into an atoll, the process being so slow that the coral is 

 always built up to the surface, whereas that which passes 

 below the 25-fathom line ceases to grow, owing to death 

 of the animals. Other theories hold that a deeply- submerged 



