120 CORAL-REEFS. 



of which not one rises above that height, to which the 

 waves and winds in an open sea can heap up matter. 



On what foundations, then, have these reefs and islets 

 of coral been constructed ? A foundation must originally 

 have been present beneath each atoll at that limited depth, 

 which is indispensable for the first growth of the reef- 

 building polypifers. A conjecture will perhaps be hazarded, 

 that the requisite bases might have been afforded by the 

 accumulation of great banks of sediment, which owing to 

 the action of superficial currents (aided possibly by the 

 undulatory movement of the sea) did not quite reach the 

 surface, — as actually appears to have been the case in some 

 parts of the West Indian Sea. But in the form and dis- 

 position of the groups of atolls, there is nothing to 

 countenance this notion ; and the assumption without any 

 proof, that a number of immense piles of sediment have 

 been heaped on the floor of the great Pacific and Indian 

 Oceans, in their central parts far remote from land, and 

 where the dark blue colour of the limpid water bespeaks 

 its purity, cannot for one moment be admitted. 



The many widely-scattered atolls must, therefore, rest on 

 rocky bases. But we cannot believe that the broad summit 

 of a mountain lies buried at the depth of a few fathoms 

 beneath every atoll, and nevertheless throughout the 

 immense areas above-named, with not one point of rock 

 projecting above the level of the sea; for we may judge 

 with some accuracy of mountains beneath the sea, by those 

 on the land ; and where can we find a single chain several 

 hundred miles in length and of considerable breadth, much 

 less several such chains, with their many broad summits 

 attaining the same height, within from 120 to 180 feet? 

 If the data be thought insufficient, on which I have 

 grounded my belief, respecting the depth at which the 



