eA es 
L. V. Pirsson—Phonolitic Rocks from Montana. 395 
Thickly scattered everywhere between these large pseudo- 
leucites are small white or pinkish-colored crystals which run 
from 1 to 2™™ in diameter, also spherical in form but which 
occasionally show sections characteristic of the dodecahedron. 
These are sodalites. 
These two minerals, together with a rare occasional feldspar, 
_ tabular in form and 2 or 3™™ long, form the only phenocrysts. 
The rock is so thickly spotted with them that as a result of the 
contrast of their white color with that of the dark ground-mass 
it has a strongly mottled appearance that is extremely charac- 
teristic. 
On weathered surfaces. the pseudo-leucites assume a_ pale 
brown color and show a zonal weathering, in that they exhibit 
an outer shell about 1™™ in thickness, which is succeeded by a 
thin shell of more spongy material. On fresh surfaces this 
outer zone may also be seen by a slightly different color and 
luster from the rest of the crystal. 
The sodalites on weathered surfaces turn brown and disap- 
pear easily, leaving pits in their former places. 
When the rock is powdered and treated with acid it gelati- 
nizes so readily and strongly as to show a very large proportion 
of high alkali silicates present. The solution in nitric acid 
yields an abundant precipitate with silver nitrate, indicating a 
large amount of sodalite in the rock. 
Microscopic.—In thin section the microscope discloses the 
following minerals as present: Titanite, egirite-augite, egirite, 
sodalite, orthoclase, nephelite, fluorite, calcite and several 
unknown minerals; the pseudo-leucite is a mixture of ortho- 
clase and nephelite. 
Pseudo-leucite.— Under the microscope these are seen to be 
made up in general of an allotriomorphic mosaic of orthoclase 
and nephelite. In some cases the outer edge of the section is 
composed of orthoclase crystals with their longer axes per- 
pendicular to the crystal faces of the original leucite. They 
form in fact the outer shell previously mentioned. Such an 
arrangement is similar to that of pseudo-leucites from Brazil 
and Magnet Cove, as noted by Graeff* and J. Francis Williams.t 
Inside of this shell, which is rarely perfect, the usual granitoid 
mixture occurs. At times the orthoclase laths in the center of 
the pseudo-leucite are grouped around some common point, 
forming a sphero-crystal, and a number of these may occur in 
the same pseudo-leucite. Again at other times the orthoclase 
contains small streaks and patches of nephelite in such a way 
as to give rise to a distinct micro-pegmatitic structure. 
* Jahrb. f. Min., 1887, vol. ii, p. 257. 
+ Arkansas Geol. Surv., Ann. Rep., 1890, p. 268. 
