1836.] BREACHES IN BARRIER-REEFS. 477 
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 gate- 
ways 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 surprise at the reefs 
both of atolls and barriers becoming in parts imperfect. The 
great barrier of New Caledonia is thus imperfect and broken in 
many parts; hence, after long subsidence, this great reef would 
not produce one great atoll 400 miles in length, but a chain or 
archipelago of atolls, of very nearly the same dimensions with 
those in the Maldiva archipelago. Moreover, in an atoll once 
breached on opposite sides, from the likelihood of the oceanic 
and tidal currents passing straight through the breaches, it 
is extremely improbable that the corals, especially during con- 
tinued subsidence, would ever be able again to unite the rim , 
if they did not, as the whole sank downwards, one atoll would be 
divided into two or more. In the Maldiva archipelago there are 
distinct atolls so related to each other in position, and separated 
by channels either unfathomable or very deep (the channel be- 
tween Ross and Ari atolls is 150 fathoms, and that between the 
north and south Nillandoo atolls is 200 fathoms in depth), that 
