297 



the process by which this change is accomplished, Mr. Kirwan, be- 

 lieving 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 

 particles 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 car- 

 tilaginous part, he thinks the following change may ensue, on their 

 being long left in a humid situation. The earthy part will separate, 

 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, &c. 

 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. 

 VOL. I. Q Q 



