s8o Count de Bgurnon's Description of 
of those of the primitive rhomboid : they are produced by the 
passing of the crystal to a less obtuse pyramidal modification. 
Corundum is not the first mineral substance that has exhi- 
bited, even in its crystallized state, very striking differences, 
according to the circumstances that have governed its forma- 
tion, and the greater or less degree of perfection that has taken 
place in its crystallization. Felspar is a substance to which 
the very same remarks may be applied. In the interior part ‘of 
most kinds of granite and porphyry, it appears in the form of 
very rugged crystals, generally opaque ; whereas, in the fissures 
of primitive rocks, it frequently has a beautiful transparency ; 
and, when this happens, it rather exceeds the former kind in 
hardness and in gravity. This difference, which for a long time 
prevented the latter kind from being joined with the felspar 
of granites, is so striking, that most naturalists have thought it 
right still to continue to separate it, at least as a variety, al- 
though they allow it a place in the same genus, under the name 
of a dul aria. 
There exists also in the same genus (felspar) a third variety, 
which, though it had long been known by the name of white 
schorl of Dauphiny , was not, till lately, brought into its pro- 
per place. This kind of felspar, which is still more perfect, 
presents, in such of its crystals as have the greatest degree of 
transparency, a brilliancy that is even superior to that of the 
most perfect adularia ; this transparency is less similar to that 
of glass, and approaches nearer to that which is peculiar to the 
stones that have been hitherto distinguished by the names of 
gems or precious stones. Indeed, it always appeared to me to 
possess, in general, the two characters of hardness and gravity, 
in a somewhat greater degree than adularia. It rather scratches 
