384 NOTICES OF NEW BOOKS. 



side, and the fragments become in time cemented into a solid mass, 

 either made up of partially rounded fragments of corals of various 

 sizes, or of calcareous sandstone, formed of a conglomerate in which 

 the structure of the' coral has often become much obscured or 

 entirely lost. 



The interior of the lagoon is the next subject that requires 

 notice. In the case before us, about half its area consists of 

 sediment and half of coral reefs ; but the corals are very different 

 from those on the outside, and are much more delicate. The reefs 

 also are softer, and increase very rapidly. 



The sediment from the deepest parts in the lagoon consists of 

 calcareous sand, and large soft banks of similar mud have been ob- 

 served in other atolls. When dry, this mud so closely resembles 

 chalk, that it can scarcely be distinguished from that substance. 

 Large shoals of fishes, and many other animals, are found to 

 feed upon the branches of the living coral, and the excrement of 

 these must constantly produce vast quantities of fine calcareous 

 mud. 



Judging from old charts of Keeling Atoll, the coral polyp has 

 added much to the solid matter of the reef within a very moderate 

 period, and the islets have also greatly increased ; but so far is this 

 from being the consequence of any general elevation of the land, 

 that there is good evidence in proof of a small subsidence. At any 

 rate, there is evidence here of a severe struggle in progress be- 

 tween the nicely balanced powers of land and water, and if left 

 undisturbed, it is manifest, that though the islets may increase, 

 and the lagoon become partly filled up, the general increase sea- 

 wards, and even the final conversion of the lagoon into land, must 

 necessarily be exceedingly slow. 



Keeling Atoll may be taken as a fair example of the very 

 numerous rings of coral reefs and coral islands in the Pacific and 

 Indian Oceans ; but many of them are much larger, and some, on 

 the other hand, exceedingly small, and the nature of the sub- 

 marine slope is, in the great majority of cases, similar. But al- 

 though such is the case, generally, there are two exceptions so 

 remarkable, that some special notice is due to them. These ex- 

 ceptions consist of the reefs which form the Maldive Archipelago, 

 and those which compose the great Chagos Bank. 



The Maldive Archipelago is 470 miles in length, with an 

 average breadth of about 50 miles, the dimensions of the largest 

 atoll are 88 miles by 20, and like the other larger ones, it is 

 breached by numerous deep-water channels leading into the la- 

 goon. The separate portions of the reef between these channels 

 are, in many cases, especially in the more northern atolls, true 

 ring-shaped atolls differing only from those in the open sea by 

 being based on a shallow foundation, and grouped closely together 

 on one large platform, with the marginal rings, forming the ex- 

 ternal atoll, arranged in a rudely formed circle. The lagoons in 

 the atolls of this large group are from 10 to 20 fathoms deeper in 

 the southern than in the northern part. It is observed in this 



