similar to that of the leaf, from which the impression was origi* 

 nally derived. The separated flake of bitumen, mentioned in the 

 first case, has in some instances been preserved ; the description 

 which has been given of it, it being said to resemble a piece of 

 leather or parchment, agreeing much better with its possessing a 

 bituminous nature, than its being a mere dried leaf. 



In objection to the proposed explanation of the change, which, 

 the vegetable matter undergoes in these nodules, it will occur to 

 many, that the bituminization being performed, under circumstances > 

 so favourable to it, there is reason to expect, that the process would 

 pass on to its completion, and that sometimes jet, and the darker 

 bitumens, might be thus formed. It is therefore necessary to state,, 

 that this does sometimes happen. Fig. 8,. in .Plate -I. furnishes an 

 instance of this circumstance. It appears to be the remains of a 

 tuberculated branch of some tree of the pine kind, which has been 

 involved in a nodule of iron-stone, the projecting parts of which 

 are covered with a jet-like bitumen. 



The fossil leaf has been said, in many instances, to have been 

 detached from the surface to which it adheres. This is said to have 

 been done by Lhwydd, Buttner, Henkel, Jussieu, and others; but 

 that it was a bituminous, and not a vegetable substance, which was 

 thus found, is evident from their relations. Volckman, who also 

 thus detached them, states, that he found that the earthy particles 

 of the leaves had been deprived of their aqueous and volatile parts ; 

 and that, in the place of these, a bituminous matter had insinuated 

 itself*. Walch also remarks, that these leaves should rather be said 

 to be mineralized, than petrified ; since the deficiency arising from 

 the escape of the aqueous and volatile parts is, in his opinion, sup- 

 plied by the addition of bitumen, which he considers as a subject 

 of the mineral kingdom. 



* Silesia Subterranea, p. 105. 



