Ch. V. OF CORAL-REEFS. 145 



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 

 the rate of upward growth of the polypifers, the death 

 of the reef must ensue, and it would have been strange 

 had we found no evidence of this. It is, then, not 

 at all improbable that the corals should sometimes 

 perish either on the whole or on part of a reef. 

 If only on a part, the dead portion, after a small 

 amount of subsidence, would still retain its proper 

 outline and position beneath the water. After a more 

 prolonged subsidence, it would form, owing to the 

 accumulation of sediment, a more or less level bank 

 marking the limits of the former lagoon. Such dead 

 portions of a reef would generally lie on the leeward 

 side, 1 for the impure water and fine sediment are 



1 Sir C. Lyell, in the first edition of his Principles of Geology, offered 

 a somewhat different explanation of this structure. He supposes that 

 there has been subsidence ; but he was not aware that the submerged 

 portions of reef were in most cases, if not in all, dead ; and he attributes 

 the difference in height in the two sides of most atolls, chiefly to the 

 greater accumulation of detritus to windward than to leeward. But as 

 matter is accumulated only on the backward part of the reef, the front 



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