352 Weed and Pirsson — Bearpaw Mountains^ Montana. 



ridges and crests about the stock. The rocks of the contact 

 zone are well exposed on the south side of the core, where they 

 form the cliffs on either side of Beaver Creek, dipping up 

 stream and away from the stock at 20°, which becomes but 15° 

 farther away from the core. Detached masses of the altered 

 sedimentary rocks are in one place found resting upon the 

 granular rock, as shown in fig. 4. On the crest of the spur 

 showing these detached masses the line of contact is seen to be 

 somewhat irregular, and the igneous rock breaks through the 

 baked sedimentaries in chimney -like masses. The western 

 border of the stock shows the best exposures of the contact 

 zone. Above the uniform crest of a ridge of massive rock, the 

 slopes of altered sedimentary strata rise abruptly to a high 

 ridge forming the divide between the waters of Beaver Creek 

 and the stream to the west. The ridge is formed of highly 

 altered, hardened, metamorphosed shales forming adinoles of 

 white and light creamy tones of brown, green, lavender and 

 pink, together with sandstones altered to dense compact quartz- 

 ites, the beds dipping at 20° away from the stock. The 

 metamorphic influence of the igneous rock is noticeable in a 

 zone about a half mile in width. 



The massive rock shows considerable variation in character. 

 A rather finely granular, even-grained, syenitic rock showing 

 equal proportions of feldspathic and of ferromagnesian min- 

 erals, forms the main body of the mass, but this passes rapidly 

 in some places into a dark, highly basic type, and on the north- 

 ern boundary a, highly feldspathic variety is seen, a syenite, 

 which as it is the most acidic type occurring at this core will 

 be considered first. 



Beaver core syenite. — This variety occurs on the north side 

 of a branch creek, from the east, near a miner's cabin. It 

 forms a debris pile of rudely platy blocks, is hard and tough, 

 ringing under the hammer, and breaking with difficulty. The 

 blocks are lichen-covered, but the rock is quite fresh though 

 stained with iron teachings near the surface. On a freshly 

 fractured surface the rock is seen to be fine grained, evenly 

 granular, and compact. The color is gray, slightly stained 

 with iron rust. The rock is essentially feldspathic, and of 

 somewhat aplitic aspect. Close examination shows certain 

 feldspars developed with a tabular habit which give long 

 cleavage planes with parallel arrangement. This and the 

 " Schlieren " seen in the field show a rude flow-structure. 

 Examined with the lens, it is seen to be composed mainly of 

 light-colored feldspar, with a few inconspicuous small spots of 

 green pyroxene. 



