THE FOSSILIFEROUS ROCKS. 25 



cemented together by 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 for- 

 mations of the earth's crust. These are seen, on weathered or 

 broken surfaces, or still better in polished slabs (fig. 9), to be com- 



Flg. 9. Slab of Crinoidal marble, from the Carboniferous limestone of Dent,ln 

 Yorkshire, of the natural size. The polished surface intersects the columns of the 

 Crinoids at different angles, and thus gives rise to varying appearances. (Original.) 



posed 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 lime- 

 stones of bygone ages. Again, we find other limestones such 

 as the celebrated " Nummulitic Limestone" (fig. 10), which 

 sometimes attains a thickness of some thousands of feet which 

 are almost entirely made up of the shells of Foraminifera. In 

 the case of the " Nummulitic Limestone, " just mentioned, these 



