341 
is formed by a thin cortical coat, exactly resembling that of the 
inner cortical investment of recent wood. Another form, which, in 
these instances, the external surface sometimes bears, is that of 
apparently dried, and withered wood, as at Plate 11. Fig. 9- Both in 
this, and in the other specimen, the seeming cortical surfaces are 
pervaded by silex, but appear to be but little changed by bitumini- 
zation. This may assist, in accounting for their not possessing the 
resinous lustre, which distinguishes the rest of the mass : numerous 
observations seeming to point out, that the bark, and dry withered 
wood, are not so susceptible of bituminous change, as those parts in 
which the woody fibre was unimpoverished. 
Another circumstance, hitherto inexplicable, demands next to be 
noticed. In many specimens of silicious fossil wood, are found 
disposed, in and about it, distinct masses of a substance, bearing 
so exactly the appearance of bitumen, as would leave no doubt of 
its being of that nature, were it not that examination discovers it to 
be a substance, containing a considerable proportion of silex. The 
explanation of the formation of this mysterious substance is, un- 
doubtedly accompanied with considerable difficulty. After having 
examined it in every point of view, I discover no mode of explaining 
its formation, but by supposing it to have been actually a fluid dark 
bitumen, which, by the medium of the water we have already seen it 
contains, has become impregnated by silicious earth. 
The general appearance of this seeming bituminous substance 
appears strongly to point it out, as having existed in a soft state . 
some pieces of it appearing as if their eminences had been rounded 
off by slight friction. In the specimen figured at Plate II. Fig. 4. 
which has very much the appearance of a piece of fossil deal wood, 
this black bituminous -like matter almost covers one end, and pe- 
netrates into the substance of the wood, as may be seen by the 
section of the same specimen, at Fig. 5. of the same Plate : the 
wood having split at the end, as if from dryness ; and the bitumen, 
