CORAL-REEFS. 47 



Where there is deep water, for instance above twenty 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. Mr. Lyell 1 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 its circumference. Several facts indicate that a con- 

 siderable quantity of sediment is likewise discharged through 

 these channels ; and Captain Moresby informs me that he 

 has observed, during the change of the monsoon, the sea 

 discoloured to a distance off the entrances into the Maldiva 

 and Chagos atolls. This, probably, would check the growth 

 of the coral in them, far more effectually than a mere 

 current of water. In the many small atolls without any 

 channel, these causes have not prevented the entire ring 

 attaining the surface. The channels, like the submerged 

 and effaced parts of the reef, very generally though not 

 invariably occur on the leeward side of the atoll, or on that 

 side, according to Beechey, 2 which, from running in the 

 same direction with the prevalent wind, is not fully exposed 

 to it. Passages between the islets on the reef, through 

 which boats can pass at high water, must not be confounded 

 with ship-channels, by which the annular reef itself is 

 breached. The passages between the islets occur, of course, 

 on the windward as well as on the leeward side ; but they 

 are more frequent and broader to leeward, owing to the 

 lesser dimensions of the islets on that side. 



At Keeling atoll the shores of the lagoon shelve gradually, 

 where the bottom is of sediment, and irregularly or abruptly 



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



2 Beechey's Voyage^ 4to ed., vol. i. p. 189. 



