ch. y. 
or coral-reefs. 
147 
merely of dead coral-rock ; that there are barrier-reefs 
and atolls with only a portion of the reef, generally on 
the leeward side, submerged; and that such portions 
either retain their perfect outline, or appear to be 
more or less completely effaced, their former place 
being marked only by a bank, conforming in general 
outline with that part of the reef which remains 
perfect. These several cases are, I believe, intimately 
related, and can all be explained by the same agency 
of subsidence. 
"We see that in those parts of the ocean where 
coral-reefs are most abundant, one island is fringed 
and another neighbouring one is not fringed, and that 
in the same archipelago, all the reefs are more perfect 
in one part than in another, —for instance, in the 
southern compared with the northern half of the 
Maldiva Archipelago, and likewise on the outer as 
compared with the inner coasts of the double row of 
atolls in this same archipelago. The existence of the 
innumerable polypifers forming a reef depends on 
their finding sustenance, and we know that they are 
preyed on by other organic beings, and that some 
inorganic causes are highly injurious to their growth. 
Can it, therefore, be expected that the reef-building 
polypifers should keep alive for perpetuity in any one 
place, during the round of change to which earth, 
air, and water are subjected ; and still less can this 
be expected during progressive subsidence, to which 
by our theory these reefs and islands have been liable ? 
Should such subsidence be at any time greater than 
