274 BKEACHEri IN 15ARRIER-REFFS. 



by MM. Quoy and Gaimard were applicable, not 

 to reefs in general, as implied by them, but only 

 to those of the fringing-class ; my surprise, how- 

 ever, ceased when I afterwards found that, by a 

 strange chance, all the several islands visited by 

 these eminent naturalists could be shown, by their 

 own statements, to have been elevated within a 

 recent geological era. 



Not only the grand features in the structure of 

 barrier-reefs and of atolls, and of their likeness to 

 each other in form, size, and other characters, are 

 explained on the theory of subsidence — which 

 theory we are independently forced to admit in 

 the very areas in question, from the necessity of 

 finding bases for the corals within the requisite 

 depth — but many details in structure and excep- 

 tional cases can thus also be simply explained. I 

 will give only a few instances. In barrier-reefs it 

 has long been remarked with sui-prise, that the 

 passages through the reef exactly face valleys in 

 the included land, even in cases where the reef is 

 separated from the land by a lagoon-channel so 

 wide and so much deeper than the actual passage 

 itself, that it seems hardly possible that the very 

 small quantity of water or sediment brought down 

 could injure the corals on the reef Now, every 

 reef of the fringing-class is breached by a narrow 

 gateway in front of the smallest rivulet, even if dry 

 during the greater part of the year, for the mud, 

 sand, or gi-avel, occasionally washed down, kills 

 the corals on which it is deposited. Consequently, 

 when an island thus fringed subsides, though most 

 of the narrow gateways will probably become closed 

 by the outward and upward growth of the corals, 

 yet any that are not closed (and some must always 

 be kept open by the sediment and impure water 

 flowing out of the lagoon-channel) will still con- 



