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previous bituminization, the silicious impregnation, in those spe- 
cimens which I possess, has been such as to have given to them suf- 
ficient hardness to dispose to the conchoidal fracture, approaching 
in some parts to the shivery ; and to occasion them to yield sparks, 
by percussion with steel. Their external surface has sometimes the 
whiteness of wood which has been long exposed to the air; but a 
section of them shows the internal substance to be of the colour of 
dark brown bituminized wood. The character of this species of 
fossil wood is, that although the silicious fluid has pervaded every 
part of it, it has entered into no other union with it, than that of 
penetrating every fibre, and filling up every minute interstice, 
without entering into such chemical union with the bitumen, as to 
allow it to manifest the least trace of bitumen, in its lustre, trans- 
parency, or colour, in any of those parts where the silex has been 
allowed to transude, and separate from the general mass of the im- 
pregnated wood : the silicious matter assuming, generally, in those 
situations, the mammillated surface, and that degree of transparency, . 
which is in general displayed by silicious matter, in that form which 
is termed mammillated calcedony. In PI. II. Fig. 10, is represented 
a specimen, showing the whitened surface, beautifully invested with 
transparent mammillated calcedony, appearing as if it had perco- 
lated through the substance, and had hardened on the surface of 
the wood. Plate VIII. Fig. 9, represents another specimen, in 
which the circular cavities, which had been formed by the teredo 
navalis, are filled up by the transudation of the uncoloured, pellucid 
calcedony. At Plate II. Fig. 11, a specimen is depicted, which 
appears to have suffered a higher degree of bituminization, in conse- 
quence of which the filtered silicious matter, with which it is everv 
where beset, in the form of yellow semi-pellucid globules, appear to 
have derived some slight tinge from the bitumen ; which, in conse- 
quence of the more perfect state of bituminization, in which it had 
