24 PRINCIPLES OF PALEONTOLOGY. 



This is the case, for example, with the so-called " Crinoidal 

 Limestones " and " Encrinital Marbles " with which the geolo- 

 gist 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 

 (hg. 9), to be composed more or less exclusively of the broken 



ida' marble, from the Carboniferous limestone of Dent, in York- 

 The polished surface intersects the columns of the Crinoids at 

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



V.g 9 Slab ot L 

 sh re, of the natural 



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 simi- 

 larly 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 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 shells are of 

 large size, varying from the size of a split pea up to that of a 



