Sect. I. KEELING ATOLL. 17 



parts reddish from ferruginous matter : it is very hard 

 and sonorous under the hammer : it is obscurely divided 

 by seams, dipping at a small angle seaward : it consists 

 of fragments of the corals which grow on the outer 

 margin, some quite and others partially rounded, some 

 small and others between two and three feet across ; and 

 of masses of previously formed conglomerate, torn up, 

 rounded, and recemented : or it consists of a calcareous 

 sandstone, entirely composed of rounded particles 

 of shells, corals, the spines of echini, and other organic 

 bodies generally almost blended together ; — 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 calcareous matter ; and I col- 

 lected an 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, from three to four 

 fathoms, and smaller basins from eight to ten fathoms 



c 



