418 WEED AND PIRSSON 
orthoclase in the preceding rock. Anorthoclase is probably 
largely present. The feldspars have been slightly kaolinized — 
the groundmass, which is similar in character to 920, much more 
so. No ferro-magnesian mineral was observed save an occa- 
sional tiny fiber or shred of egirine and almost no iron ore; the 
rock consists almost entirely of alkali feldspars and is practically 
a sanidinite-porphyry. It belongs clearly in the alkali series. 
Shellrock Butte is a round-topped eminence, separated by a 
deep saddle from Granite Mountain to the east and an equally 
low divide from the main mountains to the west. The lower 
slopes are formed of metamorphic schists which show consider- 
able variety, including sheared granite, garnet schists, feldspar 
schists, and black mica and hornblende schists. The saddle 
- west of this mountain is formed by the head waters of a branch 
of Lodgepole Creek and of Ruby Creek. The schists extend 
upward some two or three hundred feet above the saddle. 
Above this point they are covered by the porphyry débris, which 
hides the contact and obscures the exact relationship of the 
two rocks. 
Granite Butte is the name applied to the most striking summit 
and highest elevation of the eastern end of the range. Gentle 
northerly slopes are in abrupt contrast with a bold craggy sum- 
mit, abrupt cliffs, and steep rock débris slopes on the south. It 
is probably the highest point of the mountains, and consists of 
a granite-porphyry which is somewhat different in appearance 
from the rock prevailing generally throughout the range. The 
rock is open in structure and weathers in great blocks that lie 
piled one upon another like rude masonry. 
The rock is a granite-diorite-porphyry. \t is a somewhat com- 
pact rock of a light-gray color, characterized by large crystals 
of glassy orthoclase which are sometimes 20™™ across, and small 
phenocrysts of opaque white feldspar. Round grains of glassy 
gray quartz are abundant, and vary in size up to5™™. The 
rock also carries stout prisms of chloritized augite, which on the 
weathered surface have left cavities giving the rock a pitted 
appearance that is quite noticeable. 
