LI. V. Pirsson—Phonolitic Rocks from Montana. 397 
masses and stringers which are occasionally clearly visible to 
the naked eye. It occurs very commonly at times in the center 
of the large pseudo-leucite crystals or in vein-like masses fill- 
ing cracks from the outer boundary. It has the usual unequal 
distribution of its violet pigment. The occurrence of the min- 
eral renders it probable that its origin is due to pneumatolytic 
processes. 
In the center of the pseudo-leucites there frequently occur 
small crystals of an unknown mineral. Its habit is that of a 
rather stout prism which rarely exceeds ‘5™™ in length and is 
usually much smaller. It is bounded by well defined crystal 
planes. Examined with a lens in the hand-specimen, they are 
seen to be of a brown color with a resinous luster. It has a 
good cleavage parallel to one of the long prism planes. Placed 
on this cleavage face under the microscope the following 
optical properties appear. The extinction is parallel to the 
prism edge and a positive bisectrix emerges in the center of 
the field, the plane of the optic axes being parallel to the long 
prism edge. There is a not very strong pleochroism parallel to 
the prism edge = a = brown, at right angles = 6, a yellow. It 
was at first thought that the mineral might be astrophyllite, 
and three small crystals, all that could be obtained, were tested 
for MnO and TiO.,, for the former in a sodium carbonate bead, 
which was then dissolved in a minute quantity of H,SO, and 
tested for TiO, with hydrogen peroxide. No reaction was 
obtained in either case, though a nearly equal quantity of astro- 
phyllite from Brevig treated in the same way gave excellent 
reactions for both. The small solution of the mineral was 
then tested for iron, which was present. The mineral is not 
astrophyllite, both on chemical and optical grounds. 
If we assume that the prism edge represents the vertical 
axis and the cleavage face is a(100), then the above facts and 
those learned by a study of the sections may be summarized as 
follows: 
Orthorhombic, the pinacoids @(100) and 6(010) present with 
long tapering pyramids or domes. Cleavage parallel to a(100). 
Axial plane parallel to (010). And a=c, b=6, c=a. Color 
brown, luster resinous, pleochroism not marked, a brown, 
6 yellow, c between. Scarcely perceptible in thin section. 
Refraction high, at least 1:6. Double refraction high, at least 
04. Does not contain MnO; contains not more than a trace 
of TiO,; iron is present. : 
These properties distinguish the mineral from astrophyllite, 
lavenite, rinkite, mosandrite and cossyrite, which might be 
expected to occur ina rock of this nature. It is to be hoped 
that better and more abundant material may be discovered 
