Ch. V. 
OF CORAL-REEFS. 
151 
or breaches through the rim, were all of the same depth 
as the central expanse into which they lead ; whereas 
the channels into the other atolls of the Chagos group, 
and as I believe into most other large atolls, are not 
nearly as deep as the lagoons. For instance at Peros 
Banhos, the channels as well as the bottom of the lagoon 
for a space about a mile and a-half round its shores, are 
only between 10 and 20 fathoms in depth, whilst the 
central expanse is from 35 to 40 fathoms deep. Now, 
if an atoll during a gradual subsidence once became 
entirely submerged like the Great Chagos bank, and 
therefore no longer exposed to the surf, very little 
sediment could any longer be formed from it ; conse- 
quently the channels leading into the lagoon would be 
no longer filled up with drifted sand and coral detritus, 
and would continue increasing in depth, as the whole 
sank down. In this case we might expect that the 
currents of the open sea, instead of sweeping as at 
first round the submarine flanks, would, as the many 
breaches in the reef increased, flow directly across 
the lagoon, thus removing the finer sediment from 
the channels, and preventing its further accumulation. 
The submerged reef would thus ultimately consist of an 
upper and narrow broken rim of rock, surrounded on 
the inner side by banks, the remnants of the sandy 
bed of the old lagoon, now intersected by many deep 
channels ; these channels, with their sides worn steep by 
the oceanic currents, uniting in the centre and forming 
the central deep expanse. By such means the Great 
