CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 17 



representative classes or groups of animals, it is only in the 

 case of tlte lowly-organised Globigerince, and of some other 

 organisms of little higher grade, that we find absolutely the 

 same kinds or species of animals in both. 



Limestone, like chalk, is composed of carbonate of lime, 

 sometimes almost pure, but more commonly with a greater or 

 less intermixture of some foreign material, such as alumina 

 or silica. The varieties of limestone are almost innumerable, 

 but the great majority can be clearly proved to agree with 

 chalk in being essentially of organic origin, and in being 

 more or less largely composed of the remains of living beings. 

 In many instances the organic remains which compose lime- 

 stone are so large as to be readily visible to the naked eye, 

 and the rock is at once seen to be nothing more than an 

 agglomeration of the skeletons, generally fragmentary, of 

 certain marine animals, cemented together by a matrix of 

 carbonate of lime. This is the case, for example, with the 

 so-called " Crinoidal Limestones," and " Encrinital Marbles " 

 with which the geologist is so familiar, especially as occurring 

 in ' great beds amongst the older formations of the earth's 

 crust. These are seen, on weathered or broken surfaces, or 

 still better in polished slabs, to be composed more or less 

 exclusively of the broken stems and detached plates of sea- 

 lilies (Crinoids). Similarly, other limestones are composed 

 almost entirely of the skeletons of corals ; and such old 

 coralline limestones can readily be paralleled by formations 

 which we can find in actual course of production at the 

 present day. We only need to transport ourselves to the 

 islands of the Pacific, to the West Indies, or to the Indian 

 Ocean, to find great masses of lime formed similarly by living 

 corals, and well known to every one under the name of 

 "coral reefs." Such reefs are often of vast extent, both 

 superficially and in vertical thickness, and they fully equal 

 in this respect any of the coralline limestones of bygone 

 ages. Again, we find other limestones such as the cele- 

 brated " Nummulitic Limestone," which sometimes attains a 

 thickness of some thousands of feet to be almost entirely 

 made up of the shells of Foraminifera. In the case of 

 the " Nummulitic Limestone " just mentioned, these shells 



VOL. I. B 



