156 
THEORY OF THE FORMATION 
Ch. V. 
proportion of the exterior annular reef would consist of upright coral, 
and how much of fragmentary rock, for this would depend on many 
contingencies, — such as on the rate of subsidence occasionally allow- 
ing a fresh growth of coral to cover the whole surface, and on the 
breakers having force sufficient to throw fragments over this same 
space. The conglomerate which composes the base of the islets, 
would (if not removed by denudation together with the exterior reef 
on which it rests) be conspicuous from the size of the fragments, — ■ 
the different degrees in which they have been rounded, — the presence 
of ragments of conglomerate torn up rounded and re-cemented,— 
and from the oblique stratification. The corals which lived in the 
lagoon-reefs at each successive level, would be preserved upright, 
and they would consist of many kinds, generally much branched. 
In this part, however, a very large proportion of the rock, and in 
some cases nearly all of it, would be formed of sedimentary matter, 
being in an excessively fine or moderately coarse state, with the par- 
ticles almost blended together. The conglomerate which was formed 
of rounded pieces of the branched corals on the shores of the lagoon, 
would differ from that formed on the islets and derived from the 
outer coast ; although both might have been accumulated very near 
each other. The stratification, taken as a whole, would be hori- 
zontal : but the conglomerate beds resting on the exterior reef, and 
the beds of sandstone on the shores of the lagoon and on the ex- 
ternal flanks of the reef, would probably be divided (as at Keeling 
atoll and at Mauritius) by numerous layers dipping at considerable 
angles in different directions. The calcareous sandstone and coral 
rock would almost necessarily contain innumerable shells, echini, 
and the bones of fish, turtle, and perhaps of birds : possibly, also, 
the bones of small saurians, as these animals find their way to 
islands far remote from any continent. The large shells of some 
species of Tridacna would be found vertically imbedded in the solid 
rock, in the position in which they lived. We might expect, also, 
to find a mixture of the remains of pelagic and littoral animals in 
the strata formed in the lagoon, for pumice and the seeds of plants 
are floated from distant countries into the lagoons of many atolls; 
on the outer coast of Keeling atoll near the mouth of the lagoon, the 
shell of a pelagic Pteropodous animal was brought up on the arming 
of the sounding-lead. All the loose blocks of coral on Keeling atoll 
were burrowed by vermiform animals ; and as every cavity, no doubt, 
ultimately becomes filled with spathose limestone, slabs of the rock 
would, if polished, probably exhibit the excavations of such burrow- 
ing animals. The conglomerate and fine-grained beds of coral-rock 
would be hard, sonorous, white, and composed of nearly pure cal- 
