Ch. V. 
OF CORAL-REEFS. 
121 
tinguished from an atoll might be formed ; and I 
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 
central part : and this is an assumption without any 
evidence. 1 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 harrier-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. 
1 [The more vigorous growth of the coral on the outward part of a 
reef is, however, asserted and advocated by Mr. Murray as producing 
the atoll form. See Appendix II.] 
