58 . Ill DIST. CHAR. Mode. 



thus commenced, is continued, under appropriate 

 circumstances,, by a reciprocal removal of organized, 



is substituted in the place of an organic body, that " non tamen 

 semper omnes corporum destructorum particulae auferuntur, nam 

 destillando haud raro tales expelli possunt, quae organicorum na- 

 turam redolent." Med. de. Syst. Foss. p. 48. Hence, we find, 

 that the fact advanced by Mr, Parkinson, viz, that organic matter 

 still exists in petrifactions of the formation we are now considering, 

 is by no means a new discovery, or one incompatible, at least in 

 Bergman's opinion, with the theory of a gradual substitution. In- 

 deed,how should it be so? Intromission is merely an advancement in 

 the process of a change, commenced by impregnation (III. 4.) 

 and can seldom take place to such an extent, as to extrude every 

 particle of the original body, from the composition of the mass, 

 which is at last produced. It remains, therefore, only to be con- 

 sidered, 1. whether a complete petrifaction (that is, one in which 

 rib animal or vegetable matter can be discovered, except by the 

 aid of a chemical test) owes its general structure to the original 

 matter displaced; or, as Mr. Parkinson supposes, to that which 

 still exists, involved in the substance of tlie stone 2. whether the 

 original, previous to its impregnation, was or was not in a bitu- 

 minated state and, 3. how far the colour of the organic body 

 may have operated in producing that of its mineral substitute. 



In the first place, we must readily admit, that it is difficult to conceive 

 the manner, in which the organic materials have been carried away, 

 particle after particle, and those of a mineral origin substituted in 

 their place, so as to take the form of every vessel and fibre of the 

 destroyed body. But, on tlie other hand, is the dith'culty removed 

 by the supposition, that a proportion of the original matter, scarcely 

 discoverable by chemical means, can be diffused through a large 

 mass of stone, so as to infix in every molecule, as in petrified wood,an 

 organic structure ? We have now before us a specimen of silicified 

 wood, the minutest particle of which, when examined with a glass 



