42 
ATOLLS. 
Cfl. I. 
frequently they are quite irregular — some with per- 
pendicular, some with sloping sides— some rising 
to the surface, and others lying at all intermediate 
depths from the bottom upwards. I cannot, there- 
fore, suppose that the union of such reefs could pro- 
duce even one uniformly sloping ledge, and much 
less two or three one beneath the other, and each ter- 
minated 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 sur- 
veyed by him in the Low Archipelago, did not con- 
tain lagoons. Bomanzoff 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 surround- 
ing a muddy flat, one mile in its longest axis, which is 
1 Kotzebue’s First Voyage, vol. iii. p. 221. 
