the Corundum Stone , and its Varieties , &c. 281 
adularia than is scratched by it.' In the division which I usually 
make of the different kinds of felspar, I distinguish this latter, 
in consequence of the above-mentioned character, by the name 
of brilliant felspar. 
We shall see hereafter, that there probably exists a fourth 
variety of felspar, without reckoning that which is known by 
its greasy aspect. The fracture of this greasy kind is dull, and 
resembles that of wax. It exists, in great quantity, in certain 
granite -rocks, which usually abound with hornblende; of which 
rocks there is a great number in Scotland. In these, it is 
frequently of a green colour, which gives it exactly the appear- 
ance of jade. 1 his kind of felspar may very probably be a 
particular kind of substance, nearly allied to one of those (very 
different from each other) to which French mineralogists give 
at present the name of petrosilex. 
COMPACT CORUNDUM. 
We have hitherto seen corundum only in a form more or less 
perfect or determined ; it is, however, sometimes met with in a 
state in which there does not appear the smallest rudiments of 
crystallization. In this state, (to express which, mineralogists 
have agreed to make use of the term compact,) it resembles, in 
many respects, a coarse jasper ; but its much greater degree of 
hardness, and its much higher specific gravity, render its true 
nature easily distinguishable. 
In Mr. Greville's collection are many specimens of this 
compact corundum; they are all of a purplish red colour, not 
very deep, and are perfectly opaque. By means of a lens, there 
