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sits, in their places, the stony particles, which are necessarily dis- 
posed in the same order, and in the same form, with those of the 
ligneons parts, which have been removed; since they have been 
distributed in the same points, and have been moulded in the same 
cavities, which the molecules of the decayed wood have left. All 
the substance of the Avood is thus, by degrees, remoA^ed, and its 
place exactly filled by a stony substance, bearing the exact appear- 
ance of the wood itself. These stones, then, he says, are, in fact, 
not petrifactions, but only stony depositions which have received 
the impressions of different parts of the wood ; and he concludes, 
that there is no A^egetable substance which can become petrified ; 
and that petrifaction can only take place in animal substances, of 
which a part already possesses a stony hardness. To prevent any 
mistake on this subject, he observes, that petrified wood should pos- 
sess the distinctive character of wood, by possessing the medullary 
productions. It is not sufficient to see concentric layers, it is ne- 
cessary there should be also lines traversing these annual layers ; as 
they are beheld in the transverse section of a tree, from the pith to 
the bark, and in some trees, as the cork and the green oak, even in 
the bark itself 
The opinion of Mons. Fourcroy, on this subject, very much resem- 
bles that of Mons. Daubenton. The layers, he says, of fossil wood, 
penetrated by Avater, lose a portion of their dissoluble, mucilaginous, 
and extractive matter, with a part of the hydrogen, Avhich they 
contained. Hence they approach to the state of a pure wooden 
skeleton; the external substance of the woody fibres being even 
decomposed, and taking on the colour and appearance of charcoal. 
It appears, he says, in fact, that, although still Avoody, the fossij 
wood is approaching to destruction ; and that a still longer stay in 
the earth would destroy it entirely. With respect to petrified wood. 
* Lecons Normales, tom. iii. Observations sur les Petrifications. 
