Sect. II. ATOLLS. 39 



the existence of more deeply submerged banks now 

 possessing little or no signs of their original atoll-like 

 structure, are probably the effects of a uniform cause, — 

 namely, the death of the coral, during the subsidence of 

 the area, in which the atolls or banks are situated. 



There are seldom (with the exception of the Maldiva 

 atolls), more than two or three channels, and generally 

 only one leading into the lagoon, of sufficient depth for 

 a ship to enter. In small atolls, there is usually not 

 even one. Where there is deep water, for instance 

 above 20 fathoms, in the middle of the lagoon, the 

 channels through the reef are seldom as deep as the 

 centre, — it may be said that the rim only of the saucer- 

 shaped hollow forming the lagoon is notched. Sir C. 

 Lyell * has observed that the growth of the coral would 

 tend to obstruct all the channels through a reef, except 

 those kept open by discharging the water, which during 

 high tide and the greater part of each ebb is thrown 

 over a large portion of its circumference. Several facts 

 indicate that a considerable quantity of sediment is 

 likewise discharged through these channels ; and 

 Captain Moresby has observed, during the change of 

 the monsoon, that the sea is discoloured to some dis- 

 tance off the entrances into the Maldiva and Chagos 

 atolls. This would probably check the growth of the 

 coral in the channels, far more effectually than if they 

 merely discharged a current of water. Where there 

 is not any channel, as in the case of many small atolls, 

 these causes have not prevented the entire ring attain- 



1 Principles of Geology, vol. iii. p. 289. 



