148 THEORY OF THE FORMATION Ch. V. 



like ledges of dead coral-rock which border trie lagoons 

 of certain atolls, but their much greater width, and 

 their being formed of sand, are points of essential dif- 

 ference. On the eastern side of the atoll some of the 

 banks are linear and parallel, like islets in a great river, 

 and they point directly towards a great breach on the 

 opposite side of the atoll : these are best seen in the 

 large published chart. I inferred from this circum- 

 stance, that strong currents sometimes set directly 

 across this great bank ; and I hear from Captain Moresby 

 that this is the case. I observed, also, that the channels, 

 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 







