Troost on the Pyroxene. 
63 
is found in the Basalt of the neighbourhood of Cologne, 
seems to belong to this variety. The properties of this 
substance differ much from the former ; it is amorphous, 
black with a tinge of green and a resinous lustre — it is 
opaque, or sometimes slightly translucent. Its sp. gr. is 
It melts with intumescence before the blow-pipe, 
into a black or brown glass. According to Klaproth it 
contams 
Silex, 55 
Alumine, 16.50 
Magnesia, 1.75 
Lime,- 10 
Ox. of iron, 13.75 
Manganese, trace 
Water, 1.50 
98.50 
It is this analysis which has given it a place in the 
Pyroxene family. It is besides found in the form of 
small fragments in a bed of Carb. of Lime at Ginliana in 
Sicily. Dolomieu found it in Val di Noto of a greyish 
sireeri or brownish colour. 
o 
4. Mtered Volcanic P. This Pyroxene offers three 
modes of alteration, which it is of interest to point out. 
When the lavas have been exposed a long time to the ac- 
tion of the sulphureous acid vapours, which rise continu- 
ally from the solfutara and the craters of the volcanos, their 
nature is changed, and this is also the case with the Py- 
roxene which is imbeded in these lavas. The Pyrox- 
ene becomes opaque, white, and the constituent parts, 
with exception of the Silex, enter into combination 
with the acid vapours, forming soluble salts, which are 
