358 
Grains. 
Silex 93.125 
Alumine 1.625 
Volatile inflammable parts, and water 5.250 
100 *. 
The opinion which I have ventured to offer, respecting the tran- 
sition, if I may be thus allowed to employ the term, of bitumen, 
into the noble opal, appears to be corroborated, and the fact to be 
illustrated, by another modification of bitumen and silex, in which 
the union of these two substances appears to be incontestible. In 
the specimens in which this combination occurs, and which evidently 
are of vegetable origin, the external surface presents evident marks 
of wood, which appears to have undergone but a slight degree of • 
bituminization. This ligneous coat is, in some parts, not more than 
a sixteenth of an inch in thickness, and in some parts is so thin, as 
merely to give the appearance of a film ; adhering to a flinty matter, 
of which the substance is formed. This substance differs in no 
respect, in its appearance, from common flint (gun-flint, feuer- 
stein), except in being more dull and opaque ; as if it contained a 
portion of dark-coloured bitumen, or petroleum : this appearance 
being rather more conspicuous, towards the external surface; it 
gradually assuming the appearance of common flint ; and, acquiring 
a greater degree of pellucidity and transparency, as it reaches the 
interior surface ; and, in one specimen, it there assumes the form 
and appearance of a calcedonic stalactite. 
The film, or coat, bearing indisputable marks of ligneous origin, 
whilst the rest of the mass appears to be formed by the blending of 
bitumen and silex, offers an appearance, not to be explained, without 
some difficulty. An analogous circumstance is frequently observ- 
* Analytical Essays, p. 444. 
