305 



said, to have been converted into an agate, to the depth only of half 

 an inch ; the inner parts being slightly petrified, and the central 

 still wood *. What authority is due to this account, I pretend not 

 to determine : but must acknowledge, that no circumstance, which 

 I have arrived at the knowledge of, would have induced me to have 

 expected such an event. If the fact be so, it not only is an addi- 

 tional answer to the reasonings of Mr. Playfair, but proves, as Mr. 

 Kirwan observes, that silicious particles are soluble in water, are 

 taken up by wood, and that petrifaction is carried on, under ap- 

 propriate circumstances, in modern times -f. 



Having sketched the outlines of those hypotheses, by which the 

 process of the petrifaction of vegetable matter has been endea- 

 voured to be explained ; and having also stated those circumstances 

 which induce me to hesitate, at admitting their sufficiency, I shall, 

 in my next, venture to offer a different explanation of this process, 

 and shall then proceed to examine, whether it agrees any better 

 with the appearances which these substances yield. 



Yours, &c. 



* Juste Gesch. des Erdkorpers, 267. I Gerh. Gcsch. 222. 

 t Geological Essays, p. 140. 



VOL. I. RR 



