Sect. I. 
KEELING ATOLL. 
19 
expansions, especially in the deeper basins of the 
lagoon. The reefs on which these corals grow are 
very irregular in form, are full of cavities, and have 
not a solid flat surface of dead rock, like that surround- 
ing the lagoon ; nor can they be nearly so hard, for 
the inhabitants by the aid of crowbars made a channel 
of considerable length through these reefs, in which a 
schooner, built on the S.E. islet, was floated out. It is 
a very interesting circumstance, pointed out to us by 
Mr. Liesk, that this channel, although made less than 
ten years before our visit, was then, as we saw, almost 
choked up with living coral, so that fresh excavations 
would be absolutely necessary to allow another vessel 
to pass through it. 
The sediment from the deepest parts in the lagoon, 
when wet, appeared chalky, but when dry, like very fine 
sand. Large soft banks of similar, but even finer 
grained mud, occur on the S.E. shore of the lagoon, 
affording a thick growth of a Fucus, on which turtle 
feed ; this mud, although discoloured by vegetable 
matter, appears from its entire solution in acids to be 
purely calcareous. I have seen in the Museum of the 
Geological Society, a similar but more remarkable sub- 
stance, brought by Lieut. Nelson from the reefs of 
Bermuda, which, when shewn to several experienced 
geologists, was mistaken by them for true chalk. On 
the outside of the reef much sediment must be formed 
by the action of the surf on the rolled fragments of 
coral ; but, in the calm waters of the lagoon, this can 
take place only in a small degree. There are, however, 
