82 
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 conservation of fossil shells and analogous 
reliquiae, 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 belemnite is always known by its radiated structure, and the 
gryphite has retained its original lamina 1 . But the nature of the im- 
bedding 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 recognized 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 
