Any mineral matter being introduced into this mould, ac- 

 quires, as it hardens, a renewal or redintegration of the ex- 

 ternal form of the original substance. This cast is some- 

 times hollow, having been formed by crystallizations which 

 have only invested the inner surface, but have not been suf- 

 ficient to fill the cavity of the mould. 



The casts or nuclei of shells, fruits, reeds, &c. require 

 to be examined to ascertain whether the substance of the 

 fossil is similar or not to that of the matrix in which it is 

 found. If it is similar, the cast may be considered as having 

 been simultaneous in its formation with the bed in which it 

 exists : if not, its substance may have been yielded by per- 

 colation and subsequent crystallization in the mould ; or it 

 may have been formed in some former bed, on the breaking 

 up of which it may have been deposited among the materials 

 of the succeeding rock in which it is now found. 



Vegetable or animal substances deprived of life, and ex- 

 posed to the action of the air, in combination with moisture, 

 are rapidly decomposed; the softer and more volatile of 

 their constituent principles are dispersed under new forms, 

 and their more solid parts, such as wood and bone, are some- 

 times left cohering and displaying the form of the original 

 skeleton. When in this state, deprived of those principles 

 which conduce to chemical changes, these substances 

 become capable of continuing unaltered in their forms, in 

 different situations, for comparatively very long periods. 

 But either vegetable or animal substances, when placed, 

 under the influence of moisture, in situations excluding the 

 access of atmospheric air, undergo certain peculiar chemical 

 changes. 



