April, 1836. theory of subsidence. 



557 



reef is not attached quite close to the shore. This appears 

 to be the result of two causes : namely, first, that the water 

 immediately adjoining the beach is rendered turbid by the 

 surf, and therefore inj arious to all zoophytes ; and, secondly, 

 that the larger and efficient kinds only flourish on the outer 

 edge amidst the breakers of the open sea. The shallow space 

 between the skirting reef and the shore has, however, a 

 very different character from the deep channel, similarly 

 situated with respect to those of the encircling order. 



Having thus specified the several kinds of reefs, which 

 differ in their forms and relative position with regard to the 

 neighbouring land, but which are most closely similar in all 

 other respects (as I could show if I had space), it will, 

 I think, be allowed that no explanation can be satisfactory 

 which does not include the whole series. The theory which 

 I would offer, is simply, that as the land with the attached 

 reefs subsides very gradually from the action of subterranean 

 causes, the coral-building polypi soon raise again their solid 

 masses to the level of the water : but not so with the land ; 

 each inch lost is irreclaimably gone ; — as the whole gradually 

 sinks, the water gains foot by foot on the shore, till the last 

 and highest peak is finally submerged. 



Before I explain this view more in detail, I must enter on 

 a few considerations, which render such changes of level not 

 improbable. Indeed, the simple fact of a large portion of 

 the continent of South America, still rising under our eyes, 

 and abounding with proofs of similar elevations on a grander 

 scale during the recent period, takes away any excessive 

 improbability of a movement similar in kind, but in an 

 opposite direction. Mr. Lyell, who first suggested the idea 

 of a general subsidence with reference to coral reefs, has 

 remarked that the existence of so small a portion of land in 

 the Pacific, where so many causes both aqueous and igneous 

 tend to its production, renders such sinking of the founda- 

 tion probable. There is, however, another argument of 

 much greater weight, which may be inferred from the incon- 

 siderable depth at which corals grow. We see large extents 



