Ch. V. 
OF CORAL-REEFS. 
157 
eareous matter; in some few parts, judging from the specimens at 
Keeling atoll, they would probably contain a small quantity of iron. 
I have seen a conglomerate now forming on the shores of the Mal- 
diva atolls, resembling conglomerate limestone from Devonshire. 
Floating pumice and scoriae, and occasionally stones transported in 
the roots of trees (see my Naturalist’s Voyage, p. 461) appear the 
only sources through which foreign matter is brought to coral-for- 
mations standing in the open ocean. The area over which sediment 
is transported from coral-reefs must be considerable; Captain 
Moresby informs me that during the change of monsoons, the sea 
is discoloured to a considerable distance off the Maldiva and Chagos 
atolls. The sediment off fringing and barrier coral-reefs must be 
mingled with the mud which is brought down from the land, and is 
transported seaward through the breaches which occur in front of 
almost every valley. If the bed of the ocean were to be upraised 
and converted into land, the atolls of the larger archipelagoes would 
form hat-topped mountains, varying in diameter from a few to sixty 
miles — for the smallest atolls would probably be worn quite away ; 
and from being horizontally stratified and of similar composition, 
they would, as Sir C. Lyell has remarked, falsely appear as if they 
had originally been united into one vast continuous mass. Such 
great strata of coral-rock would rarely be associated with erupted 
volcanic matter, for this could only take place, as may be inferred 
from what follows in the next chapter, when the area in which they 
were situated, commenced to rise, or at least ceased to subside. 
During the enormous period necessary to effect an elevation of tho 
kind just alluded to, the surface would necessarily be greatly de- 
nuded ; hence it is highly improbable that any fringing-reef, or even 
any barrier-reef, at least those encircling small islands, would be 
preserved to a distant period. From this same cause, the strata 
which were formed within the lagoons of atolls and the lagoon- 
channels of barrier-reefs, and which must consist in a large part of 
sedimentary matter, would more often be preserved to future ages, 
than the exterior solid reef composed of massive corals in an upright 
position ; although it is on this exterior part that the present exist- 
ence and further growth of atolls and barrier-reefs depend. 
12 
