435 
of the leaf, from which the impression was originally 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. 
