CORAL-REEFS. 149 



external flanks) 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 the islands far remote from 

 any continent. The large shells of some species of Tridacna would be 

 found vertically imbedded n 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 case 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 lime- 

 stone, slabs of the rock taken from a considerable depth, would, if 

 polished, probably exhibit the excavations of such burrowing animals. 

 The conglomerate and fine-grained beds of coral-rock would be 

 hard, sonorous, white and composed of nearly pure calcareous 

 matter; in some few parts, judging from the specimens at Keeling 

 atoll, they would probably contain a small quantity of iron. Floating 

 pumice and scoriae, and occasionally stones transported in the root of 

 trees (see my Journal of Researches, p. 549) appear the only sources, 

 through which foreign matter is brought to coral-formations standing in 

 the open ocean. The area over which sediment is transported from 

 coral-reefs must be considerable : Capt. 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 of 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 atolls of 

 the larger archipelagoes were upraised, the bed of the ocean being 

 converted into land, they would form flat-topped mountains, varying in 

 diameter from a few miles (the smallest atolls being worn away) to 

 sixty miles ; and from being horizontally stratified and of similar com- 

 position, they would, as Mr. 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 



