136 Weed and Firsson — Bearpaw Mountains^ Montana. 



Aet. XYI. — The JBearpaw Mountains of Montana. Second 

 Paper ;* by Waltee Haevey Weed and Louis Y. Piesson. 



[Continued from page 362 of vol. i.] 



In our first paper on this interesting mountain group in tlie 

 April and May numbers of the preceding volume of this 

 journal, we described the extrusive igneous rocks and those 

 forming the stock at the head of Beaver Creek. In the pres- 

 ent communication we propose to describe several other 

 intrusive masses, a leucite lava from Bearpaw Peak, and finish 

 with an account of some of the dikes occurring so abundantly 

 throughout the region. 



Augite Syenite of Wind Butte. 



There are several other igneous centers in the Bearpaw 

 Mountains besides that of Beaver Creek, for the altered sedi- 

 mentary rocks of the contact zones of such intrusive masses 

 were noted at several localities, though but one other of these 

 stocks was visited. This intrusion occurs at the head of Wind 

 Creek, just north of Wind Butte, the highest point of the 

 northern part of the mountains. The presence of an igneous 

 center in this vicinity is clearly indicated by the large number of 

 radial dikes observed cutting the ridges about the intrusive, 

 as well as by the indurated and altered character of the sedi- 

 mentaries which they cut. The sedimentary beds about this 

 core are tilted at considerable angles, flattening in dip as the 

 distance from the stock increases, so that the intrusion is seen 

 to have tilted the sedimentary strata as at the Beaver Creek 

 core. 



In a basin at the extreme head of Wind Creek the massive 

 granular rock is seen, forming a talus slope above the springs 

 in which the stream heads. The rock is an augite syenite. 

 It has a clear gray color, is of rather fine, even grain and 

 highly feldspathic ; very little of any dark-colored component 

 is visible. With a lens it is seen to be evenly though quite 

 sparsely sprinkled with very small black augites. An occa- 

 sional cleavage of feldspar appears by reflected light to be 

 much larger than the average size of grain, which is about 1°^°^. 

 It resembles the quartz syenite of the Beaver Creek core in 

 structure, granularity, and appearance. 



Under the microscope the composition is seen to be very 

 simple, cegirite-augite and microperthite being the chief com- 

 ponents, with a little accessory iron ore, apatite^ hiotite, tita- 

 nite, hornblende., and sodalite. 



* Published by authority of the Director of the United States Geological Sur- 

 vey. Field observations and collections by W. H. W., petrography by L. Y. P. 



