60 . III. DIST. CHAR. Mode. 



The substances, most subject to this mode of sub- 

 stitution, are wood, corals, and other bodies, whose 

 texture is particularly porous Petrifactions of 

 shells, leaves, &c. are rarely the production of this 

 process. 



The matter of the petrifactions which are formed 

 by intromission, is usually siliceous calcareous 



their originals; but this, in numberless instances, is evidently not 

 the case; and, where the tint of the fossil actually resembles that 

 of the recent subject, the agreement appears to be merely accident- 

 al, and not to depend on a retention of the original, colorific par- 

 ticles, as Mr. Parkinson seems to infer. We are not, however, 

 disposed to deny, that a very small proportion of bituminous mat- 

 ter may give, in some cases, a tinct to t!;e stone in which it is in- 

 corporated; but, that the general colour of the mass, when it re- 

 sembles that of recent wood, is in reality derived from the same 

 source, cannot, we think, be admitted. To this we may add, that 

 the variety of shades and lines, by which the annual circles, the 

 medullary insertions, and the whole ligneous structure, are marked, 

 in the fossil, as distinctly, though not with the same colour, as in 

 the original wood, is well accounted for by the theory of a gradual 

 substitution ; which supposes the softer or medullary fibres to de- 

 cay, generally, faster than the more woody or durable, and that 

 the latter infiltrations may differ materially from those of the mat- 

 ter deposited in the first stages of the process and, of course* 

 mark with a variety of tint the structure of the parts which they 

 imitate. 



On the whole, therefore, we see no reason to reject a 

 theory so generally received, for that which Mr. Parkinson has en- 

 deavoured to introduce. At the same time, we beg leave to 

 state the great respect we entertain for that gentleman's chemical 

 knowledge, as well as for his ingenuity displayed in the investiga- 

 tion of the present subject. 



