42 ATOLLS. Ch. I. 



shall hereafter see that the theory which accounts for 

 the ordinary form of atolls, apparently includes this 

 occasional peculiarity in their structure. 



In the midst of a group of atolls, there sometimes 

 occur small, flat, very low islands of coral formation, 

 which probably once included a lagoon, since filled 

 up with sediment and coral-reefs. Captain Beechey 

 entertains no doubt that this has been the case with 

 the two small islands, which a]one of thirty-one sur- 

 veyed by him in the low Archipelago, did not con- 

 tain lagoons. Romanzoff Island (in lat. 15° S.) is 

 described by Chamisso 1 as formed by a dam of madre- 

 poritic rock inclosing a flat space, thinly covered with 

 trees, into which the sea on the leeward side occasion- 

 ally breaks. North Keeling atoll appears to be in a 

 rather less forward stage of conversion into land : it 

 consists of a horse-shoe shaped strip of land surrounding 

 a muddy flat, one mile in its longest axis, which is 

 covered by the sea only at high-water. When de- 

 scribing South Keeling atoll, I endeavoured to show 

 how slow the final process of filling up a lagoon must 

 be ; nevertheless, as all causes do tend to produce this 

 effect, it is very remarkable that not one instance, as I 

 believe, is known of a moderately sized lagoon being 

 filled up even to the low-water line at spring-tides, 

 much less of such a one being converted into land. It 

 is, likewise, in some degree remarkable, how few atolls, 

 except small ones, are surrounded by a single linear 



1 Kotzebue's First Voyage, vol. iii. p. 221. 



