,Ch. V. OF COKAL-KEEFS. 151 



spaces supposed to have subsided, have been raised 

 above the level of the sea, — as whole regions are now 

 rising, for instance, in Scandinavia and South America, 

 — and as no reason can be assigned why subsidence 

 should not have occurred in some parts of the earth's 

 crust on as great a scale as elevation, this objection has 

 little force. The remarkable point is, that a subsiding 

 movement to such an extent and amount should have 

 taken place within a period, during which the corals 

 have continued to add matter to the same reefs. An- 

 other and less obvious objection to the theory may 

 perhaps be advanced, namely, that, although atolls and 

 barrier-reefs are supposed to have gone on subsiding 

 for a long period, yet that their lagoons and lagoon- 

 channels have only rarely come to exceed 40 and 

 never 60 fathoms in depth. But if our theory is worth 

 consideration, we already admit that the rate of sub- 

 sidence has not ordinarily exceeded that of the upward 

 growth of the massive corals which live on the margins 

 of the reefs, so that we have only further to suppose 

 that the rate has never exceeded that at which lagoons 

 and lagoon-channels are rilled up by the growth of the 

 delicate corals which live there, and by the accumula- 

 tion of sediment. As the filling-up process, in the case 

 of barrier-reefs lying far from the land, and of the larger 

 atolls, must be an extremely slow one, we are led to 

 conclude that the subsiding movement has always been 

 equally slow. And this conclusion accords well with 

 what is known of the rate of recent movements of 

 elevation. 



