Sect. II. ATOLLS. 35 



Chamisso describes Y 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 Grarcia 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 Captain 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 Grroup, 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 at 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 the Maldiva atolls 

 this is very conspicuous, and likewise in some of the 

 Caroline and Marshall Islands. In the former, large 

 spaces consist of sand and soft clay ; and Kotzebue 



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



2 See also Moresby on the Northern Atolls of the Maldivas, Geogra- 

 phical Journal, vol. v. p. 400. 



d 2 



