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the process by which this change is accomplished, Mr. Kirwan, 
believing petrifaction to be thus performed, applies the term, sub- 
stitution ; and describes it as — “ the introduction of stony, and 
sometimes of metallic substances, into organic bodies, whether of 
the vegetable, or of the animal kingdoms, in proportion as the par- 
ticles of these organic substances are destroyed by putrefaction, so 
as to assume the place, and, consequently, the form and figure, of 
these, as if cast in the same mould. The mineral substances, thus 
moulded, are,” he says, “ in the most proper sense of the word, 
called PETRIFACTIONS*.” 
Mons. Daubenton is of opinion, that petrifaction only takes place 
in those bodies, whose composition is such, that a part of them 
already possesses a stony hardness. Such, he remarks, are the 
crustaceous coverings of some animals, shells, bones, scales, mad- 
repores, &c. In these substances, composed of a stony, and of a 
cartilaginous part, he thinks the following change may ensue, on 
their being long left in a humid situation. The earthy part will sepa- 
rate, and fall away from the cartilaginous part ; and if the lapidific 
juice meet with a body in this state, the earthy particles will enter 
into the cavities thus left in the cartilaginous part, and, by filling 
them, accomplish the petrifaction in such a manner, as to give the 
body the appearance of being still organized. 
With respect to the apparent petrifactions of wood, fruits. See. he 
supposes them to be formed in the moulds which have been left, by 
those substances, in the surrounding earth, the external surface of 
these bodies retaining the exact impression of the surrounding 
mould. But sometimes, he observes, we discover the marks of the 
internal structure of the wood; and to account for this, he adopts, 
with Mr. Kirwan, the idea of substitution. Here, he says, the water 
carries away, gradually, the ligneous fibres, and as gradually depo- 
* Geological Essays, by Richard Kirwan, Esq. p. 137. 
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VOL. I. 
