122 CORAL-REEFS. 



this at once solves every difficulty, 1 for we may safely infer, 

 from the facts given in the last chapter, that during a 

 gradual subsidence the corals would be favourably circum- 

 stanced for building up their solid frameworks and reaching 

 the surface, as island after island slowly disappeared. Thus 

 areas of immense extent in the central and most profound 

 parts of the great oceans, might become interspersed with 

 coral-islets, none of which would' rise to a greater height 

 than that attained by detritus heaped up by the sea, and 

 nevertheless they might all have been formed by corals, 

 which absolutely required for their growth a solid foundation 

 within a few fathoms of the surface. 



It would be out of place here to do more than allude to 

 the many facts, showing that the supposition of a gradual 

 subsidence over large areas is by no means improbable. 



1 The additional difficulty on the crater hypothesis before alluded to, 

 will now be evident ; for on this view the volcanic action must be sup- 

 posed to have formed within the areas specified a vast number of craters, 

 all rising within a few fathoms of the surface, and not one above it. 

 The supposition that the craters were at different times upraised above 

 the surface, and were there abraded by the surf and subsequently 

 coated by corals, is subject to nearly the same objections with those 

 given at the bottom of the last page; but I consider it superfluous to 

 detail all the arguments opposed to such a notion. Chamisso's theory, 

 from assuming the existence of so many banks, all lying at the proper 

 depth beneath the water, is also vitally defective. The same observa- 

 tion applies to an hypothesis of Lieut. Nelson's {Geolog. Trans. , vol. 

 v. p. 122), who supposes that the ring-formed structure is caused by a 

 greater number of germs of corals becoming attached to the declivity, 

 than to the central plateau of a submarine bank: it likewise applies to 

 the notion formerly entertained (Forster's Observ.^ p. 151), that lagoon- 

 islands owe their peculiar form to the instinctive tendencies of the 

 polypifers. According to this latter view, the corals on the outer 

 margin of the reef instinctively expose themselves to the surf in order 

 to afford protection to corals living in the lagoon, which belong to 

 other genera, and to other families ! 



