1856.] CONTINENTAL EXTENSION. 77 



supposed that a continent was indicated by the groups of 

 .atolls. It is difficult to guess, as it seems to me, the 

 amount of subsidence indicated by coral reefs ; but in such 

 large areas as the Lowe Archipelago, the Marshall Archi- 

 pelago, and Laccadive group, it would, judging from the 

 "heights of existing oceanic archipelagoes, be odd, if some 

 peaks of from 8000 to 10,000 feet had not been buried. Even 

 .after your letter a suspicion crossed me whether it would be 

 fair to argue from subsidences in the middle of the greatest 

 oceans to continents ; but refreshing my memory by talking 

 with Ramsay in regard to the probable thickness in one vertical 

 line of the Silurian and carboniferous formation, it seems there 

 must have been at least 10,000 feet of subsidence during these 

 formations in Europe and North America, and therefore 

 during the continuance of nearly the same set of organic 

 beings. But even 12,000 feet would not be enough for the 

 Azores, or for Hooker's continent ; I believe Hooker does not 

 infer a continuous continent, but approximate groups of 

 islands, with, if we may judge from existing continents, not 

 profoundly deep sea between them ; but the argument from 

 the volcanic nature of nearly every existing oceanic island 

 tells against such supposed groups of islands, for I presume 

 he does not suppose a mere chain of volcanic islands belting 

 the southern hemisphere. 



Fourthly. The supposed continental extensions do not seem 

 to me, perfectly to account for all the phenomena of distri- 

 bution on islands ; as the absence of mammals and Batra- 

 chians ; the absence of certain great groups of insects on 

 Madeira, and of Acacise and Banksias, &c., in New Zealand ; 

 the paucity of plants in some cases, &c. Not that those who 

 believe in various accidental means of dispersal, can explain 

 most of these cases ; but they may at least say that these 

 facts seem hardly compatible with former continuous land. 



Finally. For these several reasons, and especially con- 

 sidering it certain (in which you will agree) that we are ex- 



