CORAL-REEFS. 43 



if artificially constructed to protect the margin of Keeling 

 Island, is of frequent occurrence round atolls, I know not ; 

 but we shall presently meet with it, under precisely the 

 same form, on the outer edge of the ' barrier-reefs ' which 

 encircle the Society Islands. 



There appears to be scarcely a feature in the structure 

 of Keeling reef which is not of common, if not of universal 

 occurrence, in other atolls. Thus Chamisso describes 1 a 

 layer of coarse conglomerate, outside the islets round the 

 Marshall atolls which " appears on its upper surface uneven 

 and eaten away." From drawings, with appended remarks, 

 of Diego Garcia in the Chagos group and of several of the 

 Maldiva atolls, shown me by Captain Moresby, 2 it is 

 evident that their outer coasts are subject to the same 

 round of decay and renovation as those of Keeling 

 atoll. From the description of the atolls in the Low 

 Archipelago, given in Capt. Beechey's Voyage^ it is not 

 apparent that any conglomerate coral-rock was there 

 observed. 



The lagoon in Keeling atoll is shallow ; in the atolls of 

 the Low Archipelago the depth varies from 20 to 38 

 fathoms, and in the Marshall group, according to 

 Chamisso, from 30 to 35 ; in the Caroline atolls it is only 

 a little less. Within the Maldiva atolls there are large 

 spaces with 45 fathoms, and some soundings are laid 

 down of 49 fathoms. The greater part of the bottom in 

 most lagoons is formed of sediment; large spaces have 

 exactly the same depth, or the depth varies so insensibly, 

 that it is evident that no other means, excepting aqueous 

 deposition, could have levelled the surface so equally. In 



1 Kotzetue's First Voyage, vol. iii. p. 144. 



2 See also Moresby on the Northern atolls of the Maldivas, Geo- 

 graphical Journal, vol. v. p. 400. 



