Sect. I. 
KEELING ATOLL. 
17 
the * flat ’ are during gales of unusual violence swept 
together on the beach, where the waves each day at 
high-water tend to remove and gradually wear them 
down ; but the lower fragments are firmly cemented 
together by percolated calcareous matter, and they resist 
the daily tides longer than the loose upper fragments ; 
and thus a projecting ledge is formed. The cemented 
mass is generally of a white colour, but in some few 
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 
