CORAL-REEFS. 49 



at all intermediate depths from the bottom upwards. I 

 cannot, therefore, suppose that the union of such reefs could 

 produce even one uniformly sloping ledge, and much less 

 two or three, one beneath the other, and each terminated 

 by an abrupt wall. At Matilda Island, which offers the 

 best example of the step-like structure, Captain Beechey 

 observes that the coral-knolls within the lagoon are quite 

 irregular in their height. We 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 alone of thirty-one surveyed by him in the 

 Low Archipelago, did not contain lagoons. Romanzoff 

 Island (in lat. 15 S.) is described by Chamisso 1 as formed 

 by a dam of madreporitic rock inclosing a flat space, thinly 

 covered with trees, into which the sea on the leeward side 

 occasionally 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 describing 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- 



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



S69 



