THE DEPTHS OF THE SEA n 



indicate the great convulsions of nature 

 which are taking place in the depths of the 

 seas. 



Still another class of islands are known as 

 "coral islands," but in a way this is a mis- 

 nomer and is confusing, for most of the so- 

 called coral islands are the summits of solid, 

 rock-formed mountains which are merely 

 coated with coral. Corals of the kinds which 

 form these islands cannot live at depths 

 greater than one hundred and fifty feet and 

 hence it is impossible for corals to rear an 

 island from great depths to the surface of the 

 sea. But where there is a submerged moun- 

 tain-top or an elevation rising to within thirty 

 fathoms of the surface the corals grow upon 

 it and gradually accumulate until the summit 

 is awash. Then the action of winds and waves 

 pulverises the coral, piles it in rows and dunes 

 and gradually forms an island whose sands, 

 through countless ages, become solidified into 

 coral-limestone. Such an island may be raised 

 still further by subterranean forces and in time 

 a mass of solid coral-rock may be formed 

 which is several hundred feet above the sea. 



