28 CORAL-REEFS. 



conglomerate, torn up, rounded, and re-cemented ; or it con- 

 sists of a calcareous sandstone, entirely composed of rounded 

 particles, generally almost blended together, of shells, corals, 

 the spines of echini, and other organic bodies ; — rocks, of 

 this latter kind, occur on many shores, where there are no 

 coral-reefs. The structure of the coral in the conglomerate 

 has generally been much obscured by the infiltration 

 of spathose 1 calcareous matter ; and I collected a very 

 interesting series, beginning with fragments of unaltered 

 coral, and ending with others, where it was impossible to 

 discover with the naked eye any trace of organic structure. 

 In some specimens I was unable, even with the aid of a 

 lens, and by wetting them, to distinguish the boundaries of 

 the altered coral and spathose limestone. Many even of 

 the blocks of coral lying loose on the beach, had their 

 central parts altered and infiltrated. 



The lagoon alone remains to be described ; it is much 

 shallower than that of most atolls of considerable size. 

 The southern part is almost filled up with banks of mud 

 and fields of coral, both dead and alive; but there are 

 considerable spaces, between three and four fathoms, and 

 smaller basins, from eight to ten fathoms deep. Probably 

 about half its area consists of sediment, and half of coral- 

 reefs. The corals composing these reefs have a very 

 different aspect from those on the outside ; they are very 

 numerous in kind, and most of them are thinly branched. 

 Meandrina, however, lives in the lagoon, and great rounded 

 masses of this coral are numerous, lying quite or almost 

 loose on the bottom. The other commonest kinds con- 

 sist of three closely allied species of true Madrepora in 

 thin branches ; of Seriatafiora subulata; two species of 



1 Resembling spar. — Ed. 



