298 Weed and Pirsson — Bearpaw Mountains, Montana. 



mass in well formed crystals which average a smaller size than 

 the hornblende. Unlike the latter it is always fresh and 

 unaltered. 



The phenocrysts of feldspar are too small and occasional to 

 furnish good material for optical study. In the section they 

 are unstriated, untwinned, and have the mottled, varied appear- 

 ance between crossed nicols so often seen in anorthoclase. 

 One section of the few that were found was cut near to the 

 face J(010), the feldspar was not zonal in its growth but full 

 of irregular areas having varying birefraction and different 

 extinction angles; the direction of the base <?(001) unfortu- 

 nately could not be determined, 'but contacts with quartz show 

 that generally quartz has a higher index of refraction ; in one 

 or two areas it did not. We cannot say positively what the 

 character of the feldspar is, but we believe it to be anortho- 

 clase with included areas of an untwinned lime- soda variety. 



The phenocrysts lie in a very fine-grained ground-mass of 

 alkali feldspar in allotriomorphic interlocking grains, with here 

 and there a very little quartz. It is plentifully dotted through 

 with minute ore grains, and the feldspar is somewhat turbid 

 from kaolinization. 



The small feldspar grains of this ground-mass show charac- 

 ters similar to the larger phenocrysts ; they have the same 

 patchy, mottled appearance and are untwinned, save for an 

 occasional example after the carlsbad law. They offer only 

 confused images in convergent light, and it was found impos- 

 sible to orient and determine them. The contacts with quartz, 

 however, show that their refraction is that of the alkaline feld- 

 spars, and they are undoubtedly varying mixtures of the albite 

 and orthoclase molecules. 



Nelson intrusives. — Near Nelson P. O. there are two buttes 

 which form prominent land marks of the mountains. To the 

 south Ebony Butte is a conical summit with a black cap of 

 debris, while to the north Timber Butte rises high above the 

 valley slopes. This hill is surrounded by an annular ridge 

 with intervening hollows. The debris from the ridge shows it 

 to be a decomposed trachytic rock. A dike of what seems to 

 be the same rock forms a wall exposed on both sides of the 

 creek. The rock is a trachyte of a pale brownish-gray color ; 

 it is thickly speckled with rusty phenocrysts of a more or less 

 decomposed ferro-magnesian mineral, and occasional large tab- 

 ular plates of biotite. 



The thin section shows numerous phenocrysts of augite, 

 mica, oligoclase, and orthoclase in a very fine ground-mass of 

 alkali felkspar with a little quartz. The phenocrysts of augite 

 and biotite are almost entirely altered by decay into serpen- 

 tine, calcite, chlorite, and ferruginous products, probably 



