435 



of both its upper and under side. At some distant period, he 

 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 separated 

 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 be thereby 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 mat- 

 ter 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 

 PL III. Fig. 3, 4> and 5 ; and PI. V. Fig. 4 ; in which it may be 

 plainly seen, that the marks proceed not from impression merely, 

 not in fact from vegetable matter, but from bitumen in a soft state. 



VOL. i. 3 K 



