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. Не 
(> хх $ 
| r S 
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 
