GEOLOGY OF LITTLE ROCKY MOUNTAINS 419 
In thin section it is seen to be very much the same rock as 
that forming the central part of the mountains. Large feldspar 
phenocrysts with fewer ones of quartz in a very fine, quartzose, 
feldspar groundmass. Only an occasional patch of opacite gives 
clue to a former ferro-magnesian mineral now resorbed, perhaps 
biotite. 
The large feldspar phenocrysts are mostly of oligoclase or 
oligoclase-albite, as determined by the method of Michel Levy; 
they show both albite and Carlsbad twins and are present in thick 
tabular habits. Orthoclase, though not so prominent, is also 
largely present. A little sphene was noted. These minerals lie 
in a fine granular groundmass of micro-granitoid structure, com- 
posed of non-striated alkali feldspar and quartz. A few occasional 
granules of albite were noted in it. 
Phonolite.—A rock presenting a marked difference in character 
from those so far described was obtained from the borders of the 
porphyry mass, north of Landusky. The rock is the variety of 
phonolite called “mguaite and is found beneath the basal quartzite 
of the Cambrian, between it and the main mass of porphyry seen 
on Mill Creek above the town. The rock is a dense, dark-green 
porphyry, and at the time it was collected was supposed to be a 
contact form of the main porphyry mass. The same rock was 
found near the contact between the intruded porphyry mass of 
Indian Butte and the overlying limestones. A similar rock was 
found near the Spotted Horse mine in the Judith Mountains, 
where it also occurred at the contact between the porphyry mass 
and the altered sedimentary rocks. While definite observations 
were not made to ascertain if this rock occurs as a dike, yet the 
fact that it is found in these different localities, and in each case 
is supposed to be a contact form, seems to negative the idea that 
it is a dike rock. If, however, this rock does occur as a contact 
form of the main porphyry mass it is a most interesting occur- 
rence and tends to showa marked differentiation of the main mass 
toward the cooler periphery. The rock is quite fissile, splitting 
readily into irregularly surfaced plates, this being due to a par- 
allel arrangement of the tabular feldspar crystals. 
