Dr. Thomson’s Notice of some new Minerals. 189 
7 (ALS?) + MeS?+ KS 
Pyroxene Cal S? + (2 Mg + 1/) S? 
Amphibole Cal S* + 3 Mg S?. 
So that it differs from felspar by one-half of the potash being 
replaced by magnesia. 
2. Perthite.—'The next mineral [ have to notice I distinguish 
by the name of Perthite. It was sent me by Mr. Wilson, 
a surgeon in Perth, a township of Upper Canada; hence 
the name by which I have distinguished it. It is very much 
connected with felspar in appearance, and was sent as a va- 
riety of that mineral. 
The colour of the specimen sent me is white: it consists of 
a mass of crystals so united together as to form a kind of 
tesselated pavement. The crystals are obviously four-sided 
prisms, apparently rectangular, but not susceptible of measure- 
ment, because they cannot be isolated. 
The lustre is vitreous; the hardness is rather less than that 
of felspar ; but the specific gravity, which is 2°586, is identical 
with that of some of the varieties of that mineral. Its con- 
stituents were found to be 
RG lee tows etiaue: COs 
POPE ow ai. ys - 11°75 
Magnesia ...... 11°00 
Pretoxide of iron .. 0°225 
Moisture"... ».,. 0°65 
99°625 
From this analysis it is evident that it differs essentially from 
felspar ; the quantity of silica is much greater, and the potash 
is entirely replaced by magnesia. Its constitution may be re- 
presented by the formula 6 (Al S*)+5(Mg S‘). It isa qua- 
tersilicate, while felspar is a tersilicate. Could it be procured 
in sufficient quantity it would be an excellent material for the 
manufacture of porcelain. 
3. Peristerite*.—The next mineral which I have to men- 
tion was sent me also from Perth in Upper Canada, by Mr. 
Wilson, and also by Dr. Holmes of Montreal, under the name of 
Iridescent felspar ; but neither its characters nor its compo- 
sition correspond with that appellation. 
The specimens were amorphous masses, and had the ap- 
pearance of having constituted part of a rock blasted by gun- 
powder. 
It is light brownish red, and exhibits a play of colours, 
chiefly blue, on the surface. It is translucent on the edges ; 
* From zegioreee, a pigeon, the colours resembling a pigeon’s neck, 
