CHAPTER XI 



CORAL REEFS AND ISLANDS 



Islands may be divided into continental and oceanic, and 

 oceanic into volcanic and coral islands. When we think 

 of the innumerable coral islands and reefs of tropical seas, 

 and especially of the Pacific, and when we remember that the 

 Great Barrier Reef runs along the north-east coast of 

 AustraHa for over a thousand miles, we must realize that 

 these coral formations are amongst the greatest of oceano- 

 graphical phenomena. It is not surprising that such 

 extensive coral structures have excited the wonder and 

 curiosity of voyagers, naturalists and poets, and that many 

 fanciful speculations and scientific theories have been 

 evolved to account for the observed facts of distribution 

 and structure. 



From the earliest times navigators have noticed and 

 named three types of coral reefs : — 



The Fringing Reef, which grows along the coasts of 



continents or islands, keeping close to the shore and 



leaving no wide or open lagoon between the reef 



and the land. 



The Barrier Reef, also related to the land but at a greater 



distance, so as to leave an open navigable channel. 



The Atoll, a more or less circular ring of coral, having no 



visible relation to any land and enclosing more or 



less completely a lagoon, which may be of large extent 



and of any depth up to about 50 fathoms, usually 



much shallower. 



Islands are merely the more elevated parts of reefs which 



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