Ch. V. OF CORAL-REEFS. 125 



in the last chapter, that during a gradual subsidence 

 the corals would be favourably circumstanced for build- 

 ing 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 require 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. We have the clearest proof that a move- 

 ment of this kind is possible, in the upright trees 



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 up- 

 raised above the surface, and were there abraded by the surf and sub- , 

 sequently coated by corals, is subjected 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. Cha- 

 misso'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 observation 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 like- 

 wise applies to the notion formerly entertained (Forster's Observ. p. 

 151), that lagoon-islands owe their peculiar form to the instinctive ten- 

 dencies of the polypifers. According to this latter view, the corals on 

 the outer margin of the reef instinctively oppose themselves to the surf 

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

 to other genera and to other families ! 



