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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 
natural 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 surface, 
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 strengthened 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 tran- 
