144 CORAL-REEFS. 



sediment could be formed from it ; and consequently the 

 channels leading into the lagoon from not being filled 

 up with drifted sand and coral detritus, 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 any longer sweeping round the submarine flanks, would 

 flow directly through the breaches across the lagoon, 

 removing in their course the finer sediment, and preventing 

 its further accumulation. We should then have the sub- 

 merged reef forming an external and upper rim of rock^ and 

 beneath this portion of the sandy bottom of the old lagoon, 

 intersected by deep-water channels or breaches, and thus 

 formed into separate marginal banks ; and these would be 

 cut off by steep slopes, overhanging the central space, worn 

 down by the passage of the oceanic currents. 



By these means, I have scarcely any doubt that the 

 Great Chagos Bank has originated, — a structure which at 

 first appeared to me far more anomalous than any I had 

 met with. The process of formation is nearly the same 

 with that, by which Mahlos Mahdoo had been trisected ; 

 but in the Chagos Bank the channels of the oceanic 

 currents entering at several different quarters, have united 

 in a central space. 



This great atoll-formed bank appears to be in an early 

 stage of disseverment ; should the work of subsidence go 

 on, from the submerged and dead condition of the whole 

 reef, and the imperfection of the S.E. quarter, a mere wreck 

 would probably be left. The Pitt's Bank, situated not far 

 southward, appears to be precisely in this state ; 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 which is submerged between 5 

 and 8 fathoms. A little further south, at about the same 



