468 CIIANUES IN CORAL REEFS, [chap^. 



islands have been elevated ; and so far, this is indirecl 

 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 siiown by their own state- 

 ments 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 tlie 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 

 mto two or three atolls tied together by straight reefs — 



