CORAL-REEFS. 137 



of the innumerable polypifers forming a reef, depends on 

 their sustenance, and that they are preyed on by other 

 organic beings ; and, lastly, as we know that some inorganic 

 causes are highly injurious to the growth of coral, it cannot 

 be expected that during the round of change to which 

 earth, air, and water are exposed, the reef-building polypifers 

 should keep alive for perpetuity in any one place; and 

 still less can this be expected, during the progressive sub- 

 sidences, perhaps at some periods more rapid than at 

 others, to which by our theory these reefs and islands 

 have been subjected and are liable. It is, then, not im- 

 probable that the corals should sometimes perish either 

 on the whole or on part of a reef; if on 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 probably form, 

 owing to the accumulation of sediment, only the margin of 

 a flat bank, marking the limits of the former lagoon. Such 

 dead portions of reef would generally lie on the leeward 

 side, 1 for the impure water and fine sediment would more 

 easily flow out from the lagoon over this side of the reef, 



1 Mr. 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 

 part would remain of the same height on both sides. I may here 

 observe that in most cases (for instance, at Peros Banhos, the Gambier 

 group and the Great Chagos Bank), and I suspect in all cases, the dead 

 and submerged portions do not blend or slope into the living and 

 perfect parts, but are separated from them by an abrupt line. In some 

 instances small patches of living reef rise to the surface from the middle 

 of the submerged and dead parts. 



