506 
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 
