GEOLOGY OF LITTLE ROCKY MOUNTAINS 413 
central peak, Antoine Butte, is composed of granite-porphyry, 
normally of a gray color and distinguished by large phenocrysts 
of orthoclase. The rock usually falls apart on weathering into 
rather small angular blocks, rarely plates which are sometimes a 
foot in diameter but more often much less, the débris forming 
extensive slides and talus slopes. A specimen from the main 
crest near Antoine Butte, typical of the vicinity, is a gvanite- 
syenite-porphyry. Seenin the hand specimen the rock is compact 
and dense, of pale lavender-gray color, and shows abundant 
equidimensional phenocrysts of opaque white feldspar of 1 to 3™™ 
across, thickly scattered throughout the rock. Large crystals of 
pale flesh-colored glassy orthoclase from ro to 15™™ in length 
are less abundant, but form the most prominent constituent. 
The rock is strippled with blackish specks that are probably a 
decomposed ferro-magnesian mineral. The rock weathers with 
rusty or reddish-brown stained surface, on which the large ortho- 
clase phenocrysts are very conspicuous. 
The section discloses under the microscope an abundance of 
large phenocrysts of feldspar lying in an extremely fine-grained 
groundmass composed of quartz and feldspar. Some iron ore is 
present, dotting the section in very fine, numerous grains. No 
ferro-magnesian mineral is seen, but very rare small pseudo- 
morphs of muscovite mixed with iron ore show that formerly an 
extremely small amount of biotite in little tablets was present. 
The feldspar phenocrysts are composed of orthoclase and oli- 
goclase. The orthoclase is present in large crystals of the usual 
type showing the faces (110, (010), and c(oro), and in habit 
is stout columnar along the a@ axis. It is twinned at times 
according to the Carlsbad law and is then thick tabular on 6(010). 
A section perpendicular to the obtuse bisectrix gave an extinc- 
tion angle of about 8° from the cleavage parallel to c(oo1). The 
crystals of orthoclase are often grouped. 
The plagioclase is present in stout tabular crystals, which are 
usually not so large as those of the orthoclase. There is also 
less of it in amount. That it is oligoclase is shown by the fact 
that numerous sections oriented in the zone perpendicular to 
