359 



able in pieces of asphaltum and jet ; the bituminous change being 

 entirely completed, except at the surface, where the texture, and 

 sometimes the colour, with the other characters of the wood, will 

 be manifest. This ligneous appearance, as in the case above men- 

 tioned, is generally merely superficial ; although sometimes, indeed, 

 the fibrous appearance will extend for a quarter of an inch, or more, 

 into the mass of bitumen. The explanation which will be satisfac- 

 tory in the one instance, will also suffice for the other. We are, in 

 the first place, necessarily led to the conclusion, that, by the influ- 

 ence of some adequate cause, the surface of the wood had been 

 rendered less susceptible of the bituminous fermentation, than the 

 other part of the wood, and has therefore remained unaltered, whilst 

 the other part has completely undergone the bituminous change. 

 The causes to which this may be attributed, are either, first, a na- 

 tural disposition in the exterior surface of wood, and particularly in 

 the bark of trees, to resist the influence of the bituminous fermen- 

 tation : secondly, the previous impregnation of the external sur- 

 face, with some earthy, or metallic saline substance, which might 

 render it incapable of undergoing the bituminous change, so deep 

 as that impregnation might extend itself. This opinion is strength- 

 ened by the specimen, Plate III. Fig. 8, in which the internal part 

 is opalized, and rendered nearly transparent, whilst the external 

 part has the appearance of wood itself, and is marked externally, 

 in several parts, of a blue colour, as though it were impregnated 

 with carbonate of copper. 



In the specimens above mentioned, in which the combination of 

 dark-coloured bitumen and silex is apparent, analysis appeared to 

 be unnecessary to prove the presence of the bitumen, it was there- 

 fore omitted, especially as it could not have been accomplished 

 without injuring two valuable specimens. 



The examination of specimens of this kind, and of their transi- 



