305 
over the Danube, some miles below Belgrade. It was found, it is 
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 appro- 
priate circumstances, in modern times f. 
Having sketched the outlines of those hypotheses, by which the 
process of the petrifaction of vegetable matter has been endeavoured 
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. 1 Gerh. Gesch. 222. 
f Geological Essays, p. 140. 
VOL. I. 
R R 
