40 
ATOLLS. 
Ch. I. 
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- 
ing the surface. The channels, like the submerged and 
effaced parts of the reef, occur very generally, though 
not invariably on the leeward side of the atoll, or on 
that side, according to Beechey , 1 which, from extending 
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 irregu- 
larly or abruptly where there are coral reefs ; but this 
is by no means the universal structure in other atolls. 
1 Beechey’s Voyage, 4to ed. vol. i. p. 189. 
