Sect. I. KEELING ATOLL. 19 



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 Geolo- 

 gical Society, a similar but more remarkable substance, 

 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, other and unex- 

 pected agents at work here : large shoals of two species 

 of Scarus, one inhabiting the surf outside the reef and 

 the other the lagoon, subsist entirely, as I was assured 

 by Mr. Liesk, the intelligent resident before referred to, 

 by browsing on the living polypifers. I opened several 

 of these fish, which are very numerous and of con- 

 siderable size, and I found their intestines distended 

 by small pieces of coral, and finely ground calcareous 

 matter. This must daily pass from them as the finest- 

 sediment ; much also must be produced by the infinitely 

 numerous vermiform and molluscous animals which 

 make cavities in almost every block of coraL Dr. J* 



c 2 



