Weed and Pirsson — Bearpaw Mountains, Montana. 355 



shown to consist of over 80 per cent of alkaline feldspar of 

 varying mixtures, but whose average composition would be 

 nearly Or B Ab 7 . 



It therefore stands on the boundary in the alkali series 

 between the granites and quartz syenites; it might be termed 

 an augite aplite or granite or a quartz syenite. It is a notice- 

 able fact how high a silica content purely feldspathic rocks may 

 carry (albite with Si0 2 = 68*7^) and in this case high silica 

 may still yield little free quartz. 



Basic syenite or monzonite ( Yogoite). — This variety consti- 

 tutes the main mass of the stock. It seldom occurs in prom- 

 inent exposures, but forms talus slopes whose luxuriant cover- 

 ing of grass conceals the angular debris blocks into which the 

 rock breaks. This type occurs in different parts of the mass, 

 showing identical characters. The specimen described comes 

 from the base of the slopes east of Beaver Creek. 



The rock is of a medium gray color, is evenly and moderately 

 fine granular, the average size of grain being about l mm in 

 diameter. It is easily seen to be composed of a white felds- 

 pathic component mixed with about equal quantities of a dark 

 augite and biotite, the augite dominating the biotite in amount. 

 They do not appear porphyritic in development but in grains 

 like the feldspar ; more rarely the biotite is seen in somewhat 

 larger, ragged, poikilitic plates. 



In thin section the rock is found to be composed of the fol- 

 lowing minerals: Iron ore, apatite, diopside, biotite, soda-lime 

 feldspar varying from labradorite to oligoclase, and orthoclase. 



The diopside is a pale green, and is apt to occur in short, 

 stout prisms which have rough, rounded exteriors giving the 

 mineral an anhedral habit. 



The biotite is also allotriomorphic with respect to the other 

 minerals; it has a strong pleochroism between very pale yellow 

 and deep olive brown. Its period of formation overlaps the 

 pyroxene but commenced later ; it is very common to find the 

 pyroxenes with an interior zone filled with biotite shreds ; 

 within this it does not occur, and this marks the commence- 

 ment of the biotite crystallization. The biotite frequently 

 surrounds grains of iron ore. 



The plagioclase feldspar occurs in short, thick laths which are 

 very small in size compared with the other components, and are 

 generally quite idiomorphic. In amount it is much less than 

 any of the other components. The small laths lie scattered 

 around among the other components, and are enclosed in the 

 orthoclase without orientation. It shows carlsbad, albite, and 

 rarely pericline twinning. In composition it is somewhat vari- 

 able, the interior parts being as usual more basic ; the interior 

 part is labradorite, and the outer portion varies through 



