THE FLORIDA REEFS. 65 



islands as Barbados (Fig. 39), where the terraces formed by 

 the raised coral reefs mark the successive elevation of the vol- 

 canic cone ; or we may have still another combination, like that 

 of Guadeloupe, where a high volcanic peak forms the main 

 island, and an elevated plateau forms the Grande Terre with a 

 growing coral reef to the windward. 



The fact that these great submarine banks of modern lime- 

 stone lie in the very track of the great oceanic currents suffi- 

 ciently shows that these currents hold the immense quantity of 

 carbonate of lime needed in the growth of the bank. Its 

 amount has, besides, been actually measured by Murray. He 





Fig. 39. — Barbados Terraces. 



has shown that, if the pelagic fauna and flora extend, as the 

 experiments carried on by the " Challenger " and the " Blake " 

 seem conclusively to prove, to a depth of one hundred fathoms, 

 we should have sixteen tons of carbonate of lime for every 

 square mile one hundred fathoms deep. But the greater the 

 depth at which these plateaux begin to form, the less rapid 

 must be their formation. The fact that the deeper part of the 

 ocean, below three thousand fathoms, does not contain any of 

 the larger shells of pelagic type can be readily explained on the 

 supposition that these, being very thin, like pteropod shells, 

 present a large surface to the action of the carbonic acid, which 

 is most abundant in deep water. Attacked as soon as they 

 reach deep water by this action, these shells of thinnest surface 

 are reduced to a bicarbonate, and are carried off in solution. 

 They do not, therefore, appear at these greater depths, and are 



