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LETTER XXIX. 
OBSERVATIONS ON THE PROCESS OF PETRIFACTION. 
I ATTEMPTED, in the preceding volume, to shew that the generally 
received opinion respecting the formation of petrifactions is erroneous ; 
and endeavoured to support my conjectures by a particular examination 
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, 
molecule by molecule, as to allow the earthy parts, whilst arranging 
themselves in the spaces left by the removal of the organized matter, so 
to mould themselves into those spaces as to take exactly the form of the 
oiganized part, and to imitate it precisely, in every trace. In this 
manner, the petrifaction, as it is termed, is supposed to have acquired 
the exact form and most of the characteristic appearances of the orginal 
body, without retaining any at all of its original particles. This mode 
of explaining the formation of petrifactions has been adopted by almost 
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 explanation 
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 
became the substratum of the fossil, on which was deposited the 
lapidifying 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 
