898 L. V. Pirsson—Phonolitic Rocks from Montana. 
when the locality of this rock is found and its exact nature 
determined. 
Besides this mineral, there is another present in the rock, 
which occurs in grains and in one case in a hexagon-shaped 
section, nearly always enclosed in sodalite and associated with 
fluorite. The double refraction is extremely low and in places 
the mineral appears isotropic. It appears much like apatite, 
but the hexagonal section shows the strongest double refraction 
seen (which cannot be over -006) and does not extinguish par- 
allel to an edge. The mineral is colorless. It is thought to be 
eudialyte, but it must be admitted that the determination is very 
doubtful. 
Since the geological position of this rock is unknown, no 
analysis of it has been made. It would not differ materially 
from those already made of almost precisely similar rocks 
from other localities, and it could not be used in solving the 
general petrologic problems of the region. 
Quartz tinguaite porphyry. 
Megascopically, the rock shows a dense green ground-mass 
very similar to the type previously described, in which are very 
thickly scattered phenocrysts of feldspar. The feldspars are 
strongly tabular and somewhat columnar in habit and range 
from 1 to 13° across the flat face (6 010) and from 3 to 5™™ in 
thickness. They show by a pronounced parallel arrangement 
an excellent flow-structure in the rock. It seems probable 
that the specimen is from a dike with the feldspars arranged 
parallel to its walls, as this is the most common way in which 
rocks of this type occur. 
Microscopically, the following minerals are found present : 
Amphibole, zgirite-augite, eegirite, albite, orthoclase and quartz. 
The amphibole, which occurs in short, rather stout prisms, is 
rather rare. The prisms are from ‘5 to -2™™ in length. It has 
a low double refraction and rather large extinction angle and is 
quite pale in color. Pleochroism not marked. c, light brown- 
ish green, 6 very pale brown, a medium leather brown, absorp- 
tion a>c>b. “Apt to be altered. 
A girite-augite.—As usual, also often altered to ferruginous 
products. Enclosed in the feldspar phenocrysts, it shows its 
previous formation. 
Orthoclase.—¥From their optical properties, as shown on 
cleavage fragments, the large phenocrysts are to be referred to 
this mineral. They contain inclusions of slender lath-like 
feldspars which from their twinning, ete., are believed to be 
of albite-oligoclase; they also contain enormous quantities of 
minute microlites of a mineral of rather high refraction oceur- 
ring in grains and short prisms. This is thought to be augite. 
