110 ORGANIC REMAINS. 



The fourth condition of fossil shells, &C: is produced by a process in 

 addition to that just described. The cavity left by the removal of the 

 shell is, in this instance, filled again by crystals of carbonate of lime, in- 

 troduced by water filtrating through the stone. When this process is 

 but partially executed, the cavity is imperfectly lined with crystals, but 

 when it is completed, the new substance takes exactly the form of the 

 original shell, but displays no trace whatever of its internal structure. 

 In the Yorkshire oolites, the thick shells of trigonia and gervillia exhibit 

 this metamorphosis in a very striking manner. 



That the peculiarities in the fossil shells and analogous reliquias, 

 depend much on the original nature of the bodies, is evident from the 

 following well-known facts ; the shells and spines of echini, and the 

 columns of crinoidea are almost invariably converted to a peculiar kind 

 of calcareous spar, in whatever strata they may be found ; so the belem- 

 nite is always known by its radiated structure, and the gryphite has re- 

 tained its original laminse. But the nature of the imbedding substance 

 is also of great consequence in the inquiry. Shells which lie in the 

 green sand are generally converted to flint ; those which lie in oolite are 

 often changed to calcareous spar ; but those which lie buried in clay 

 seldom exhibit either of these characters. 



The interesting subject of the conservation of fossils might be ex- 

 tended to great length ; but as these explanations were introduced chiefly 

 to facilitate the understanding of terms which will afterwards be em- 

 ployed, there is room only for another observation on the bones and teeth 

 of vertebral animals, which are principally composed of phosphate of lime, 

 united by a cartilaginous substance. Remains of this kind being much 

 fewer than those of the preceding tribes, do not afford so great a variety 

 of mineral appearances. On the contrary, their state of preservation is 

 remarkably uniform, under whatever circumstances they are found. 

 Fish teeth, for example, are always recognised by a peculiar polish and 

 hardness, and are commonly of a black colour (except in chalk.) They' 

 retain the whole of their phosphate of lime, but part of the animal sub- 

 stance is generally replaced by an additional quantity of carbonate of 



