130 THEORY OF THE FORMATION Ch. V. 



hereafter see that the position of certain ancient build- 

 ings in the Caroline Archipelago clearly indicates recent 

 subsidence. In the chapter on Keeling atoll, I have 

 also attempted to show by direct evidence, that the 

 island subsided during the earthquakes lately felt there. 



The facts then stand as follows : — there are many 

 large spaces of ocean without any high land, inter- 

 spersed with reefs and islets formed by the growth 

 of those kinds of coral which cannot live at great 

 depths; and the existence of these reefs and low 

 islets in such numbers and at such distant points, 

 is inexplicable, excepting on the theory that their 

 rocky bases slowly and successively sank beneath the 

 level of the sea, whilst the corals continued to grow 

 upwards. No positive facts are opposed to this view, 

 and some direct evidence, as well as general considera- 

 tions, render it probable. There is also evidence of 

 change in form, whether or not from subsidence, on 

 some of these coral-islands ; and there is evidence of 

 subterranean disturbances beneath them. Will then 

 the theory, to which we have thus been led, solve the 

 curious problem, — what has given to each class of reef 

 its peculiar form ? 



Let us in imagination place within a subsiding area, 

 an island surrounded by a ' fringing-reef ' — that kind 

 of which the origin alone offers no difficulty. Let the 

 unbroken lines in the woodcut (No. 5) represent a 

 vertical section through the land and water; and the 

 horizontal shading a section through the reef. Now, as 

 the island sinks down, either a few feet at a time or quite 



