5o6 BREACHES IN BARRIER-REEFS chap. 



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 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 exceptional cases 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 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 continue to front 

 exactly the upper parts of those valleys at the mouths of 

 which the original basal fringing-reef was breached. 



We can easily see how an island fronted only on one 

 side, or on one side with one end or both ends encircled by 

 barrier-reefs, might after long-continued subsidence be converted 

 either into a single wall-like reef, or into an atoll with a great 

 straight spur projecting from it, or into two or three atolls 

 tied together by straight reefs — all of which exceptional cases 

 actually occur. As the reef- building corals require food, are 

 preyed upon by other animals, are killed by sediment, cannot 

 adhere to a loose bottom, and may be easily carried down to 

 a depth whence they cannot spring up again, we need feel no 



