CORAL-REEFS. 143 



covered with sand ; it lies at an average depth of between 

 five and eight fathoms, and perfectly resembles in form the 

 annular reef of an atoll. The banks of the second level, 

 the boundaries of which are marked by dotted lines in the 

 chart, lie from about fifteen to twenty fathoms beneath the 

 surface ; they are several miles broad, and terminate in a 

 very steep slope round the central expanse. This central 

 expanse I have already described, as consisting of a level 

 muddy flat between thirty and forty fathoms deep. The 

 banks of the second level, might at first sight be thought 

 analogous to the internal step-like ledge of coral-rock which 

 borders the lagoons of some atolls, but their much greater 

 width, and their being formed of sand, are points of 

 essential difference. On the eastern side of the atoll some 

 of the banks are linear and parallel, resembling islets in a 

 great river, and pointed 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 circumstance, 

 that strong currents sometimes set directly across this vast 

 bank ; and I have since heard from Capt. 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 

 lagoon-like space 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 their lagoons : — for instance at Peros Banhos, the 

 channels are only of the same depth, namely between 10 

 and 20 fathoms, as the bottom of the lagoon for a space 

 about a mile and a half in width round its shores, whilst the 

 central expanse of the lagoon 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 



