62 . III. DIST. CHAR. Mode. 



it may have sustained,, of its constituent principles. 

 In general,, we find transmutations of vegetable 

 bodies to yield carbonaceous, or bituminous sub- 

 stances : the animal transmutations are,, for the most 

 part calcareous ; sometimes, however, bituminous. 

 Vegetable petrifactions, under this mode, differ, 

 also, in some degree, with respect to the manner in 

 which they retain the external form of their origi- 

 nals. The mineral, resulting from the change, 



forming, in certain instances, only a thin film or 

 covering to the mass, which constitutes the chief 

 bulk of the petrifaction as in the petrified stems of 

 plants, &c. which frequently exhibit, in their struc- 

 ture, a nucleus of stone ( similar to that of the ma- 

 trix ) thinly invested with a layer or coat of coaly 

 matter. f In petrified leaves, and other like bo- 



f In describing some specimens of this kind, Mr. Parkinson has 

 so well explained the mode of their formation, that we shall take 

 the liberty of extracting a few lines from his ingenious work. 

 " Plate III. Fig. 3. represents a fossil of this kind from Chepstow, 

 in Monmouthshire. This, as well as most others of this kind, is 

 little more than an impression, covered with a bituminous film, of 

 a very inconsiderable thickness, its internal part being entirely 

 sand-stone. The explanation of this circumstance does not, how- 

 ever, appear very difficult. The plant, having been surrounded 

 by the soft or fluid materials, of which the sandstone has been 

 since formed, its internal succulent part would soon waste away, and 

 its place be filled with the soft magma; while the more solid and 

 ligneous epidermis would remain, and, after a time, would give its 

 correct impression to the surrounding lapidifying matter. Then 



