Ch. V. OF COKAL-KEEFS. 129 



the work of destruction is still in progress ; but that on 

 the other hand the first formation of some islets is 

 known to the present inhabitants. In such cases, it 

 would be exceedingly difficult to detect a gradual sub- 

 sidence of the foundation on which these mutable 

 structures rest. 



Some of the archipelagoes of low coral-islands are 

 subject to earthquakes : Captain Moresby informs me 

 that they are frequent, though not very strong, in the 

 Chagos group, which occupies a central position in the 

 Indian Ocean, and is far from any land not of coral 

 formation. One of the islands in this group was 

 formerly covered by a bed of mould, which disap- 

 peared after an earthquake, and was believed by the 

 residents to have been washed by the rain into the 

 underlying fractured rock : the island was thus ren- 

 dered unproductive. Chamisso 1 states that earth- 

 quakes are felt in the Marshall atolls, which are far 

 from any high land, and likewise in the islands of 

 the Caroline Archipelago. On Oulleay atoll one of 

 the latter, Admiral Lutke informs me that he ob- 

 served several straight fissures about a foot in width, 

 running for some hundred yards obliquely across the 

 whole width of the reef. Fissures indicate a stretching 

 of the earth's crust, and, therefore, probably changes 

 in its level ; but these coral-islands, which have been 

 shaken and fissured, certainly have not been elevated, 

 and, therefore, probably have subsided. We shall 



1 See Chamisso, in Kotzebue's First Voyage, vol. iii. p. 182 and 136. 

 K 



