LETTER XXIX. 



OBSERVATIONS ON THE PROCESS OF PETRIFACTION. 



I ATTEMPTED, in the preceding volume, to shew that the generally 

 re eived opinion, respecting the formation of petrifactions is errone- 

 ous; and endeavoured to support my conjectures by a particular ex- 

 amination of various vegetable fossils. It has been supposed, that in every 

 instance of petrifaction, the lapideous is substituted for the vegetable 

 or animal matter, as this is decomposed and removed. This removal and 

 substitution has been also supposed to be so gradually performed, mole- 

 cule by molecule, as to allow the earthy parts, whilst arranging them- 

 selves in the spaces left by the removal of the organized matter, so to 

 mould themselves in those spaces, as to take exactly the form of the 

 organized part, and to imitate it precisely, in every trace. In this man- 

 ner the petrifaction, as it is termed, is supposed to have acquired the 

 exact form, and most of the characteristic appearances, of the original 

 body, without retaining any at all of its original particles. This mode 

 of explaining the formation of petrifactions has been adopted by al- 

 most every chemist and mineralogist who has written on the subject; 

 and has been particularly described by Kirwan, Walch, Daubenton, 

 Fourcroy, and Haiiy ; the latter gentleman giving it as the explana- 

 tion which is most generally admitted, although he acknowledges that 

 it may not be free from difficulties. 



Dissatisfied with this explanation, I suggested that the organized 

 part was not removed; but that it remained in part, at least, and be- 

 came the substratum of the fossil, on which was deposited the lapidi- 

 fying matter. I endeavoured to shew that vegetable substances., 

 in certain situations, were rendered bituminous ; and were, in that 

 state, capable of being thoroughly pervaded by water, and, of 

 course, of being imbued with the saturated solution of any 

 earth ; and that by the formation of minute crystallizations through 

 the whole impregnated mass, a consolidated silicious or calcareous 



