60 Troost on the Pyroxene. 
Frascati, and found also a trace in an analysis made of a 
variety of Rhinberg, 
The regular forms which occur in this variety are the 
JBisunitary, 
Triunitary, 
Subtractivey 
Dioctsedral, 
•Ambiguous, 
Hemitropal. 
The size of the crystals is from less than one-twelfth of m 
inch to nearly an inch. 
1. Black Volcanic P. Its colour is perfectly black, or 
dark green, approaching to black. It is sometimes mag- 
netic. It melts with more difficulty than the other varieties, 
and is more brittle. Its external surface is generally rough; 
its fracture intermediate between glassy and fibrous. It oc- 
curs mostly in the true lava — in those which have run, and 
in the scoriae, which are a mere modification of the former. 
It occurs also, 1st, in Basalt; it is known that a volcanic ori- 
gin is attributed to Basalt by most mineralogists; and 2ndly, 
in transition or other rocks, of which also a volcanic orighi 
is suspected, as in the Wacke, Amygdaloides, &c. 
The rocks of Theis, near Fassa in Tyrol, which are sup- 
posed to have a volcanic origin, contain crystals of Py- 
roxene; the Klinkstein or Phonolite, a species of petro- 
siliceous lava, includes sometimes a large quantity. The 
porphyric rock of Oberstein, in which the Agate occurs, 
and those of Tyrol, which are of the same nature, include 
also the black Pyroxene. Finally, we find it sometimes in 
those substances which have been ejected unaltered by Ve- 
suvius and other volcanos. But in this case it has seldom a 
black colour, though this substance is nowhere so abundant 
