Ch. V. OP CORAL-REEFS. 149 



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 

 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 northern rim, 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 



