CORAL-REEFS. 147 



known of the rate and manner of recent elevatory move- 

 ments, of judging by analogy what is the probable rate of 

 subsidence. 



In this chapter it has, I think, been shown, that the theory 

 of subsidence, which we were compelled to receive from the 

 necessity of giving to the corals, in certain large areas, 

 foundations at the requisite depth, explains both the normal 

 structure and the less regular forms of those two great 

 classes of reefs, which have justly excited the astonishment 

 of all persons who have sailed through the Pacific and 

 Indian Oceans. But further to test the truth of the theory, 

 a crowd of questions will occur to the reader : Do the 

 different kinds of reefs, which have been produced by the 

 same kind of movement, generally lie within the same areas ? 

 What is their relation of form and position, — for instance, 

 do adjoining groups of atolls, and the separate atolls in these 

 groups, bear the same relation to each other which islands 

 do in common archipelagoes ? Have we reason to believe, 

 that where there are fringing-reefs, there has not lately been 

 subsidence ; or, for it is almost our only way of ascertaining 

 this point, are there frequently proofs of recent elevation ? 

 Can we by this means account for the presence of certain 

 classes of reefs in some large areas, and their entire absence 

 in others? Do the areas which have subsided, as indicated 

 by the presence of atolls and barrier-reefs, and the areas 

 which have remained stationary or have been upraised, as 

 shown by fringing-reefs, bear any determinate relation to 

 each other ; and are the dimensions of these areas such as 

 harmonise with the greatness of the subterranean changes, 

 which, it must be supposed, have lately taken place 

 beneath them? Is there any connection between the 

 movements thus indicated, and recent volcanic action ? All 

 these questions ought to receive answers in accordance with 



