303 



the 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 sufficient, 

 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 speci- 

 men, 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 un- 

 equally, 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, he says, that 

 the line which separates these two parts is quite sharp- and -distinct; 

 altogether different from what must have taken place* had the flinty 

 matter been introduced into the body of the wood, by any fluid in 

 which it was dissolved ; as it would then have pervaded the whole, 

 if not uniformly, 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 

 say, that I never yet saw one in which the line separating these two- 



* Illustration of the Huttonian Theory, p. 25. 



