Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.3)

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 injurious 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 gradvially 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