Weed and Pirsson — Bearpaw Mountains, Montana. 291 



extending downward in tongues cutting the grassy middle 

 slopes. The base of the butte is formed of rough outcrops of 

 purplish, reddish, and steel-colored rocks, with fragments of a 

 light-colored variety which seemed to form a dike or intrusive 

 mass, as seen from the summit of Gray Butte. 



The rock is a mica-trachyte, having a medium gray color, is 

 of fine grain and filled with many very small glittering black 

 tablets of biotite and with small whitish spots and specks. 

 Under the microscope it appears very badly altered, much 

 more so than the megascopic appearance would suggest. The 

 mica is fresh and recalls that of minettes ; there is, moreover, a 

 great deal of it, and it is strongly pleochroic and very idio- 

 morphic. 



The rock is filled with masses of calcite, which appears due 

 to the decomposition of augite ; some may possibly have been 

 introduced. Secondary quartz also appears filling cavities. 

 There are a number of phenocrysts of feldspar generally show- 

 ing both albite and carlsbad twinning ; one of these, oriented 

 in the zone 001 on 100 perpendicular to 010, gave extinction 

 angles with the Bertrand ocular for the albite twins a = 19° 

 a' = 19, the carlsbad twin gave 9°, and hence the plagioclase is 

 andesine. The ground-mass is made up of lath-shaped feld- 

 spars, which are so much altered that it cannot be safely told 

 whether a plagioclase which is present or alkali feldspar pre- 

 dominates. The alkali feldspar appears, however, to predom- 

 inate. It is certainly present, as a little interstitial quartz 

 permits the recognition by Becke's method that some less 

 altered, unstriated granules, have in all positions less refraction 

 than the quartz. The rock appears, therefore, to be a transi- 

 tion form between a mica-andesite and a mica-trachyte so far 

 as can be told. 



Gray Butte is the most western of the isolated group of three 

 elevations, which forms such a prominent feature of the east 

 end of the mountains. Unlike its neighbors it is rugged in 

 appearance, showing bold exposures of massive rock with talus 

 of large blocks covering the slopes below. The lower part for 

 200 to 300 feet above the base is a steep slope cut in soft black 

 shales that show no fossils and are apparently horizontal. 

 The shaly beds are cut at the southwestern base by a dike of 

 decomposed, hornblendic trachyte. The butte above is formed 

 of an intrusive mass of massive, alkali quartz-syenite-porphyry ', 

 breaking up through the shales in a boss a half mile wide. 

 The gray color of the outcrops and debris piles which gives the 

 name to the butte is due to lichens, as the rock is a pale-green 

 on fresh fracture, becoming pink on weathered surfaces. The 

 shales are slightly altered for a few yards distant from the 

 contact. The rock occurs in place on the summit of the butte, 



