26 
ATOLLS. 
Ch. L 
between the two nicely balanced powers of land and 
water. With respect to the future state of Keeling 
atoll, if left undisturbed, we can see that the islets 
may still extend in length ; but as they cannot resist 
the surf until it is broken by rolling over a wide space, 
their increase in breadth must depend on the in- 
creasing breadth of the reef; and this must be limited 
by the steepness of the submarine flanks, which can be 
added to only by sediment derived from the wear and 
tear of the coral. From the rapid growth of the coral 
in the channel cut for the schooner, and from the 
several agents at work in producing fine sediment, it 
might be thought that the lagoon would necessarily 
become quickly filled up. Some of this sediment, 
however, is transported into the open sea, as appears 
from the soundings off the mouth of the lagoon, in- 
stead of being deposited within it. The deposition, 
moreover, of sediment, checks the growth of coral reefs, 
so that these two agencies cannot act together with full 
effect in filling up the lagoon. We know so little of 
the habits of the many different species of corals which 
form the lagoon-reefs, that we have no more reason for 
supposing that their wdiole surface -would grow up as 
quickly as the coral did in the schooner-channel, than 
for supposing that the whole surface of a peat-moss 
would increase as quickly as parts are known to do in 
holes, where the peat has been cut away. These 
agencies, nevertheless, tend to fill up the lagoon ; but 
in proportion as it becomes shallower, so must the 
polypifers be subject to many injurious agencies, such 
