502 



CHARLES DARWIN 



tidal currents of an open ocean; whereas, we have in the 

 earthquakes recorded by the natives on some atolls, and in 

 the great fissures observed on other atolls, plain evidence of 

 changes and disturbances in progress in the subterranean 

 regions. 



It is evident, on our theory, that coasts merely fringed by 

 reefs cannot have subsided to any perceptible amount; and 

 therefore they must, since the growth of their corals, either 

 have remained stationary or have been upheaved. Now, it 

 is remarkable how generally it can be shown, by the presence 

 of upraised organic remains, that the fringed islands have 

 been elevated : and so far, this is indirect evidence in favour 

 of our theory. I was particularly struck with this fact, when 

 I found, to my surprise, that the descriptions given 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, however, 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 to 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 exceptional eases can thus also 

 be simply explained. I will give only a few instances. In 

 barrier-reefs it has long been remarked with surprise, 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 gravel, 

 occasionally washed down kills the corals on which it is 

 deposited. Consequently, when an island thus fringed sub- 



