358 



Gr. 



Silex . . .............. ..... 9 



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 incontestable. 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 mat- 

 ter, 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 arid 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. 



