BARRIER REEFS. ATOLLS. 



27 



Barrier Reefs. This name is given to reefs formed 

 as above, but distant from the shore, and separated from 

 it by deep water, as the reef, one thousand miles long and 

 thirty miles from shore, on the Australian coast. 



Fringing Reefs. These are formed near the shore, 

 generally in smooth water, having no great depth between 

 them and the adjacent land. 



Atolls. When a reef reaches the surface, the waves 

 from the side of the prevailing winds grind up the dead 



FIG. 26. Atoll in the Pacific Ocean. 



coral, and wash it over in the form of mud and sand, 

 where it settles in smooth water. Here branching corals, 

 that give shelter to innumerable animals, flourish, as well 

 as lime-secreting algae,* all continually growing and being 

 ground up by fishes and worms, and disintegrated by the 

 solvent action of the carbonic acid in the water, until 

 finally a shallow lagoon or flat is formed, wholly or partly 



* This is one of the most important factors of lagoon and key 

 making. The keys of Tortugas, especially Sand Key, are made up 

 almost entirely of these limy plates, as well as broken shells. 



