303 
aid of glasses. Here again it may with propriety be questioned, 
whether in this case the injection of silex, melted by heat, could 
possibly have taken place in this wood, without entirely destroying 
its structure, as well as colour. This breaking and dissolution, in 
some parts of a specimen, the Doctor, in the second position, con- 
siders as attributable to the fusion and crystallization of the flint. 
How this dissolution of the parts of the wood may be accounted for 
has been endeavoured to be shown above, in part ; and other causes, 
supposed to be equal to the production of the same effect, will be 
soon pointed out. In the mean time, it may be suflicient, to point 
out the difficulty of supposing, the injection of the melted flint to 
have occasioned the destruction of the vascular structure, in one 
part of a specimen ; whilst, in another part of the same specimen, 
consolidated in the same manner, the structure has suffered little 
or no injury. 
Professor Playfair states, that, “ on examination, the silicious 
matter is often observed to have penetrated the wood very unequally, 
so that the vegetable structure remains in some places entire ; and, 
in other places, is lost in an homogeneous mass of agate or jasper. 
Where this happens, it may be remarked, that the line which sepa- 
rates these two parts is quite sharp and distinct ; altogether different 
from what must have taken place, had the flinty matter been intro- 
duced into the body of the wood, by any fluid in which it was 
dissolved ; as it would then have pervaded the whole, if not uni- 
formly, yet with a regular gradation*.” 
Mr. Playfair here undoubtedly speaks of specimens which he has 
either himself seen, or of those, the description of which, he con- 
ceives, warrants this account. My objections here must be neces- 
sarily feeble, being only of a negative kind. During the perpetual 
examination, for several years, of specimens of fossil wood, I can 
♦ Illustration of the Huttonian Theory, p. 25. 
