CORAL-REEFS. 117 



on a detached bank, would tend to assume an atoll-like 

 structure ; if, therefore, corals were to grow up from a bank, 

 with a level surface some fathoms submerged, having steep 

 sides and being situated in a deep sea, a reef not to be 

 distinguished from an atoll, might be formed : I believe 

 some such exist in the West Indies. But a difficulty of the 

 same kind with that affecting the crater theory, renders, as 

 we -shall presently see, this view inapplicable to the greater 

 number of atolls. 



No theory worthy of notice has been advanced to account 

 for those barrier-reefs, which encircle islands of moderate 

 dimensions. The great reef which fronts the coast of 

 Australia has been supposed, but without any special facts, 

 to rest on the edge of a submarine precipice, extending 

 parallel to the shore. The origin of the third class or of 

 fringing-reefs presents, I believe, scarcely any difficulty, and 

 is simply consequent on the polypifers not growing up from 

 great depths, and their not flourishing close to gently 

 shelving beaches where the water is often turbid. 



What cause, then, has given to atolls and barrier-reefs 

 their characteristic forms? Let us see whether an im- 

 portant deduction will not follow from the consideration of 

 these two circumstances, — first, the reef-building corals 

 flourishing only at limited depths, — and secondly, the 

 vastness of the areas interspersed with coral-reefs and coral- 

 islets, none of which rise to a greater height above the level 

 of the sea, than that attained by matter thrown up by the 

 waves and winds. I do not make this latter statement 

 vaguely ; I have carefully sought for descriptions of every 

 island in the intertropical seas ; and my task has been in 

 some degree abridged by a map of the Pacific, corrected in 

 1834 by MM. D'Urville and Lottin, in which the low 

 islands are distinguished from the high ones (even from 



