433 
of both its upper and under side. At some distant period, be sup- 
poses, that some bituminous matter insinuated itself, and filled the 
cavity which the leaf had left ; and that, on dividing the nodule, the 
bitumen remains adherent to one side; and being sejDarated from 
the other, it exhibits the form which it has derived from the mould 
on that side, whilst the mould itself is displayed on the counterpart 
of the nodule. 
You will, doubtless, agree, if the theory proposed respecting the 
bituminization of vegetable matter accord with the phenomena 
which are yielded by the vegetable ’remains thus incarcerated in 
nodules, that considerable additional confirmation of that theory 
will thereby be produced; especially if it serves to explain satis- 
factorily those circumstances which otherwise appear so inexpli- 
cable. 
The polished surface, and sharpness of impression, observable in 
schisti bearing the characteristic traces of vegetables, manifest that 
the substance thus impressed must have been in a very soft or fluid 
state, when the impressing vegetable substance was applied to it. 
The disappearance of the impressing vegetable substance implies, 
that its resolution must have taken place, whilst excluded from the 
air by the surrounding mass ; whilst every appearance seems to point 
out, that the change which it has undergone, has been similar to that 
by which jet and the bitumens, in general, have been formed. 
The nature of the change which the leaf, or other vegetable 
matter, undergoes, whilst in this secluded state, is demonstrated still 
more plainly, on an examination of the impressions of plants on 
sand-stone ; since here the colour of the stone, which is generally 
of a light yellow or brown, allows the colour which the changed 
vegetable matter assumes to be distinctly seen. This is the case in 
PI. III. Fig. 3, 4, and 5 ; and PI. V. Fig. 4; in which it may be 
plainly seen, that the mai’ks proceed not from impression merely, 
not in fact from vegetable matter, but from bitumen in a soft state. 
3 K 
VOL. I. 
