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, which 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 

 t-ransmit light, and magnified 55 times. 



Fig. 53. 



Texture of a Tree from the Coal Strata— magnified. 



