102 RATE OF GROWTH. Ch. IV. 



sent in the deep oceans where coral formations most 

 abound. 



Nor do I think, when we consider the rate of the 

 upward growth of reefs under favourable circumstances, 

 that we should be influenced by the fact that certain 

 submerged reefs, such as those off Tahiti or those within 

 Diego Grarcia, are not now nearer the surface than they 

 were many years ago. For it has been shown that all 

 the reefs have grown to the surface in some of the 

 Chagos atolls, but that in neighbouring atolls which 

 appear to be of equal antiquity and to be exposed to 

 the same external conditions, every reef remains sub- 

 merged ; we are, therefore, almost driven to attribute 

 this to a difference, not in the rate of growth, but in 

 the habits of the corals in the two cases. 



In an old-standing reef, the corals, which greatly 

 differ in kind on different parts of it, are probably 

 all adapted to the stations they occupy, and hold their 

 places, like other organic beings, by a struggle one 

 with another and with external nature ; hence we may 

 infer that their growth would be slow except under 

 peculiarly favourable circumstances. Almost the only 

 natural condition, allowing a quick upward growth of 

 the whole surface of a reef, would be a slow subsidence 

 of the area in which it stood ; — if, for instance, Keeling 

 atoll were to subside two or three feet, can we doubt 

 that the projecting margin of live coral, about half an 

 inch in thickness, which surrounds the dead upper sur- 

 faces of the mounds of Porites, would in this case form 

 a concentric layer over them, and the reef thus increase 





