CORAL REEFS 



219 



their shelter and protection. If in this struggle for existence 

 the organisms which attack the base of the coral get the 

 upper hand over those that protect it, the time soon comes 

 when a strong wave causes the perforated base to fracture, 

 the colony topples over and is cast up into the sand of the 

 lagoon, where it is smothered or gradually falls down the 

 outer slope of the reef into deep water, to form with its com- 

 panions in misfortune a talus on which the living coral reef 

 extends. 



The broken bits of dead coral that are cast into the 

 lagoon may be further comminuted by the strong teeth of 

 many species of the coral reef fishes, by passing through the 

 alimentary canals of the holothurians and various kinds of 

 Sipunculid and Polychaet worms, and by the rolling action 

 of the surf, until, at last, they are driven on to the dry land 

 and contribute to the formation of those glistening white 

 beaches which are so characteristic of the tropical shores. 



A recent discovery by Drew ^ has shown that there is yet 

 another element entering into the complex problems of the 

 disintegration of corals and the formation of calcareous 

 sands and muds, and that is the precipitation of amorphous 

 calcium carbonate by the action of the denitrifying bacteria 

 of the sea. In the Bahamas and Florida Keys large quanti- 

 ties of a chalky mud seem to be formed by this action, and 

 it can readily be understood that if such mud, together with 

 the corals and shells which it has covered, were raised above 

 the level of the sea, it might in time become consolidated to 

 form a hard rock similar to chalk or limestone. Further 

 investigation of this important action in the Pacific and 

 Indian Oceans will doubtless lead to important results. 



The constant formation of sand and mud by the disin- 

 tegration of coral is an important factor in the determination 

 of the constitution of the reef. If it is washed away as soon 

 as it is formed the corals can thrive, but if, on the other hand, 

 it is deposited in the form of silt on any part of the living 

 reefs, the corals may be killed. 



* G. H. Drew, " On the Precipitation of Calcium Carbonate in the 

 Sea by Marine Bacteria," Papers from the Tortugas Laboratory of the 

 Carnegie Institution of Washington, vol. v., 1914. 



