3© CORAL-REEFS. 



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 unexpected agents at work here : large shoals of two 

 species of Scarus, 1 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 considerable 

 size, and I found their intestines distended by small pieces 

 of coral, and firmly 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. Allan, of Forres, who has 

 enjoyed the best means of observation, informs me in a 

 letter that the Holothuriae (a family of Radiata) subsist on 

 living coral ; and the singular structure of bone within the 

 anterior extremity of their bodies, certainly appears well 

 adapted for this purpose. The number of the species of 

 Holothuria, and of the individuals which swarm on every 

 part of these coral-reefs, is extraordinarily great ; and many 

 ship-loads are annually freighted, as is well known, for 

 China with the trepang, which is a species of this genus. 

 The amount of coral yearly consumed, and ground down 

 into the finest mud, by these several creatures, and 

 probably by many other kinds, must be immense. These 

 facts are, however, of more importance in another point of 

 view, as showing us that there are living checks to the 



1 A genus of fish, commonly called Parrot-fish, with fleshy protrusible 

 lips, belonging to the Teleostei. — Ed. 



