On Crystallography. 1 5 1 



into its interior*, and then nothing appears simpler than 

 the explanation oF the fact, by the introduction of a liquid 

 charged with stony molecules into the cavity of the shell; 

 and this observation leads to a similar explanation of the 

 formation of the kinds of nuclei modelled into shells, which 

 we meet with isolated and stripped of every envelope. 



Sometimes the shell itself has been penetrated by another 

 matter sjenerally siliceous, which has been subsiituted for 

 the cartilaginous substance of which this shell had been 

 partly composed f; and it may happen in this very cast> that 

 the interior of the shell has remained empty. It is no 

 longer, properly speaking, a pseudomorphosis. It is a fossil 

 which has merely become n)ore stony than it was before. 



This last kind of modification takes place equally with 

 respect to the bones and to the other solid pans of animals 

 which are found immured in the bowels of the cartfi ; i. e., 

 they may pass to an almost entirely stony state; hy the help 

 of a substance which supplies the place of th^^ir cartila- 

 ginous part. 



The case cannot be the same with vegetable productions 

 as with shells. They have no testudo, or envelope, which 

 can exist after the destruction of the interior substance, 

 and serve as a mould to a stony or other substance for re- 

 ceivifLig an impression of their form. If we supposed that 

 one of these productions, such as a portion of the branch 

 of a tree, were entirely destroved, so that the cavity which 

 it occupied in the bowels of the earth remained empty, we 

 could conceive that a stony matter might afterwards fill 

 thi^ cavity and there be modelled to it. In this case the 

 new body would resemble externally the branch of a tree; 

 it would have the appearance of knots and wrinkles, but 

 its inside would not present any trace of organization, and 

 it would only be, as it were, the statue of the vegetable 

 production, which ;t would have displaced. 



What is generally called petrified wood is a much more 

 faithful imitation of real wood. On a transverse section 

 we distinguish the appearance of concentric layers, which 

 in the living tree must have proceeded from its increasing in 

 thickness; all the principal lineaments of organization are 

 preserved to such a degree, that they sometimes serve tor* 

 enable us to ree6gnize the spocies to which 'the tree be- 

 longed which has undergone petrifaction. 



* De risle Crystall. tome ii. p. 161. 



f We know that shell?, as well as the bones of ani'mals,'are formed of 

 two substances; the one calcareous, whiclj is not susceptible of patrefnction ; 

 the other cartilaginous, membranous, or flashy, whicii may beflearroyed Uy 

 fcfipentation. . " " 



K 4 Among 



