1856.] CONTINENTAL EXTENSION. 435 



several reasons to suppose, were connected with mainland, as 

 Sardinia, and how different it appears. Believing, as I am 

 inclined, that continents as continents, and oceans as oceans, 

 are of immense antiquity I should say that if any of the 

 existing oceanic islands have any relation of any kind to con- 

 tinents, they are forming continents ; and that by the time 

 they could form a continent, the volcanoes would be denuded 

 to their cores, leaving peaks of syenite, diorite, or porphyry. 

 But have we nowhere any last wreck of a continent, in the 

 Tiidst of the ocean ? St. Paul's Rock, and such old batxered 

 volcanic islands, as St. Helena, may be ; but I think we can 

 see some reason why we should have less evidence of sink- 

 ing than of rising continents (if my view in my Coral volume 

 has any truth in it, viz. : that volcanic outbursts accompany 

 rising areas), for during subsidence there will be no compen- 

 sating agent at work, in rising areas there will be the additional 

 element of outpoured volcanic matter. 



Thirdly. Considering the depth of the ocean, I was, be- 

 fore I. got your letter, inclined vehemently to dispute the vast 

 amount of subsidence, but I must strike my colours. With 

 respect to coral reefs, I carefully guarded against its being 

 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 Archipelago, 

 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 

 rine 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 



