Ch. V. OF CORAL-REEFS. 119 



believe some such exist in the West Indies. But on 

 this view it must be assumed, that in every case the 

 basis consists of a flat bank; for if it were conically 

 formed like a . mountainous mass, we can see no reason 

 why the corals should spring up from the flanks instead 

 of from the central and highest parts. As the lagoons 

 of atolls are sometimes even more than 40 fathoms 

 deep, it must, also, be assumed on this view, that at a 

 depth at which the waves do not break, the coral grows 

 more vigorously on the edges of a bank than on its 

 centra] part : and this is an assumption without any 

 evidence. If we consider, moreover, the number of 

 the atolls in the midst of the Pacific and Indian 

 Oceans, this assumption of so many submerged banks 

 is in itself very improbable. 



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 evidence, to rest on the edge of a submarine 

 precipice parallel to the shore. The origin of the 

 third class, or of fringing-reefs, presents, I believe, 

 scarcely any difficulty, and arises simply from the poly- 

 pifers growing in moderate depths, and 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 important deduction does not follow from the 

 following facts, — first, that reef-building corals only 



