152 
THEORY OF THE FORMATION 
Cn. V. 
Chagos bank — the most anomalous structure which I 
have met with — appears to have originated. 
If this bank should continue to subside, a mere 
wreck of an atoll would be left ; for the corals are 
almost everywhere dead. Pitt’s bank, situated not far 
southward, appears to be in this actual condition : it 
consists of a moderately level, oblong bank of sand, 
lying from 10 to 20 fathoms beneath the surface, with 
two sides protected by a narrow ledge of rock submerged 
between 5 and 8 fathoms. A little to the south of this 
ledge, at about the same distance as the southern rim of 
the Great Chagos bank lies from the nortliernrim, there 
are two other small banks with from 10 to 20 fathoms 
on them ; and not far eastward, soundings were struck 
on a sandy bottom with between 110 and 145 fathoms. 
The northern portion of Pitt’s bank with its ledge-like 
margin, thus closely resembles any one segment of the 
Great Chagos bank between two of the deep-water 
channels, and the scattered banks southward and east- 
ward appear to be the last wreck of the less perfect 
portions of one great and now ruined atoll. 
I have examined with care the charts of the Indian 
and Pacific Oceans, and have now laid before the reader 
all the cases which I have met with, of reefs differing 
from the class to which they belong ; and I think it has 
been shown that they are all included in our theory, 
modified by occasional accidents, such as might have 
been anticipated. We have thus seen, that in the lapse 
of ages encircling barrier-reefs are converted into atolls, 
—-the term atoll being applicable as soon as the last 
