288 Weed and Pirsson — Bearpaw Mountains^ Montana. 



rough masses standing above these slopes. The rugged hills 

 forming the eastern end of the range are largely covered by 

 these crags, whose creamy tints of purple, red, brown, and 

 gray are a striking feature of the scenery. 



Within the mountain area the effusive fragmental rocks are 

 quite irregularly distributed. In general they rest upon a 

 formerly eroded surface that was hilly in nature ; so that the 

 exposures occur both as cappings to hills and in masses at 

 lower altitudes ; no definite bedding is recognizable. The 

 irregular disposition of these masses is well shown in the 

 accidented region between Clear and Beaver Creeks, where the 

 effusive rocks and sedimentary beds are exposed at various 

 altitudes and without apparent order. The rocks of this part 

 of the mountains are, in part at least, more acidic than those 

 generally prevailing, the breccias including fragments of 

 trachytic rocks. The hills in this interior part are smoothly 

 contoured, well grassed, and show the black patches of 

 gravelly debris so characteristic of these basaltic accumula- 

 tions and which accentuate and bring into strong relief the 

 grassy ridges and intervening hollows. The rocks here also 

 frequently form rough, craggy exposures, almost invariably 

 found upon the southern side of the hills. 



In the effusive rocks the breccias predominate, with associ- 

 ated tuff beds and lava flows. The rocks vary from compact, 

 dense basalts showing no porphyritical crystals, to open porous 

 forms which pass into scorias. They have a wide range of 

 colors, red, brown, gray, in warm rich tones, more rarely 

 green and buff. Though varying in appearance they consist 

 almost wholly of leucite basalts ; many varieties have no 

 megascopic phenocrysts, but most of them show a dark 

 ground-mass peppered with small white specks of altered leucite, 

 while occasionally crystals of augite are seen, and more rarely 

 olivine. Sometimes the breccias contain blocks of a dense, 

 black rock with large phenocrysts of altered mica. The tuffs 

 vary from pale-buff or dull-green to dark red, brown, or gray 

 in color, and are largely altered. 



A specimen selected for examination representing a common 

 variety seen throughout the mountains but coming from Bull 

 Hook Butte, is typical of the rock forming the larger blocks 

 and fragments in the breccias. The rock is of a very dark- 

 gray color, rather compact on the whole, though on jointing 

 surfaces it seems rather porous, and this is probably due to 

 the weathering out of an iron-bearing component ; on a 

 completely weathered surface it becomes of a leather-brown 

 color. 



On fresh surfaces one readily detects yellowish grains of 

 olivine, passing at times into a chestnut-red color and 1 or 2 



mn: 



