FOSSIL GEOLOGY OF UNITED STATES. 293 
matter into the pores of bone or vegetables. In 
some instances, the animal or vegetable matter has 
been almost dissolved or removed, and the mineral 
matter so gradually substituted, as to assume the 
perfect form of the internal structure either of the 
plant or animal. 
In some modern deposites, we find that fossil 
shells have been scarcely altered in the course of 
centuries, having simply lost a part of their animal 
matter. In other cases the shell has disappeared, 
and left an impression only of its exterior, or a cast 
of its interior form, or a cast of the whole shell, the 
original matter of which has been removed. When 
this mould is filled with mineral matter, such as 
lime, silex, iron pyrites, &c., we then have an ex- 
act cast of the shell. But there is another kind of 
petrifaction still more wonderful, which may be 
compared to certain anatomical models in wax, 
when not only the outward forms and features, but 
even the nerves, bloodvessels, and other internal 
organs, are shown. We see this in corals, and par- 
ticularly in fossil wood, where we can perceive not 
only the rings of animal growth, but all the minute 
vessels and medullary rays, the small pores of 
fibres, and even the spiral vessels, whick^can only 
be seen in a living plant by means of the micro- 
scope, as in the following cut, which represents a 
transverse slice of a fossil tree, thin enough to 
transmit light, and magnified 55 times. 
Fig. 53. 
Texture of a Tree from the Coal Strata — magnified. 
B B 2 
