Weed and Pirsson — Bearpaw Mountains, Montana. 299 



mostly limonite. The augite, indeed, has not been actually 

 seen, but its former presence is inferred from the shape of 

 pseudomorphs consisting of serpentine and calcite. The feld- 

 spars are much fresher but have also been attacked by decay, 

 and are pretty turbid from kaolin leaves. The oligoclase 

 whose presence is inferred from the constant practically par- 

 allel extinction of albite and Carlsbad twins in the zone 100 on 

 010 perpendicular to 010, is not very common and appears 

 only as a phenocryst. The orthoclase phenocrysts are much 

 more common and they appear to contain an admixture of the 

 soda molecule, as is usual in these rocks. 



The ground-mass in which the above lie is extremely fine- 

 grained, allotriomorphic, and with the exception of a little 

 residual quartz wholly of feldspar, which is untwinned and 

 an alkali feldspar and not an untwinned plagioclase, the pres- 

 ence of the quartz permitting its determination by Becke's 

 method. 



People's Creek Basin. — Just above Nelson, People's Creek 

 has cut through a sheet of dark-colored minette-like rock, 

 whose grotesque erosion forms of 3 to 4 feet in height are 

 seen standing in relief upon the grassy bench. This rock con- 

 tains a large amount of mica, making it a readily recognizable 

 type, but it yields quickly to atmospheric agencies and crum- 

 bles to a fine sandy debris. Fresh, unaltered material is very 

 hard to obtain, and although the rock is a common one in the 

 mountains good exposures of fresh rock are very rarely seen. 



In the center of People's Creek basin there is a short but 

 very striking dike-like ridge, standing abruptly above the 

 gentle, grassy, gravelly terrace slopes. The rock weathers in 

 huge blocks of pinkish or flesh color, resembling granite from 

 a distance. It is much decomposed^ and the blocks are cov- 

 ered by a thick decomposition crust so that fresh material 

 could not be obtained for examination, but it is seen to be a 

 feldspathic rock in which biotite is the only prominent pheno- 

 cryst. 



Galena Butte {Bearpaw mine). — Northwest of Nelson is 

 the low divide in the northern chain of hills, through which 

 the mail road from Lloyd has been built. East of this divide 

 there is a high summit, whose grassy open slopes crowned by 

 a log shaft house make it a conspicuous object in the landscape. 

 The building has been erected over an ore body of argentifer- 

 ous galena, in which a prospect shaft has been sunk. The ore 

 body occurs near the margin of an intrusive mass of trachyte, 

 which, as is commonly the case in these mountains, has been 

 deeply cut by the drainage, while the contact zone with its 

 denser rock and rim of hardened sediments stands out in 

 relief, forming the summit of the butte and its most prominent 

 westerly spurs. The main mass of the rock is a trachyte, gen- 



Au. Jour. Soi. — Fourth Series, Yol. I, No. 4. — April, 1896. 

 20 



