286 Weed and Pirsson — Bearpaw Mountains, Montana. 



The highest western point of the mountains is known as 

 Centennial Butte, and the isolated, conical elevation standing 

 at the extreme east end is called Eagle Butte. A few other 

 names in common use among the settlers are given upon the 

 accompanying sketch map. 



The mountains have never been surveyed. They appear on 

 most maps of the State, but only the merest approximation to 

 the drainage is given, and the intervening space is filled with 

 meaningless hachures. The accompanying sketch map, com- 

 piled from various sources and our own notes, is the best 

 obtainable. 



General Geology. 



The Bearpaw Mountains are the dissected remains of a group 

 of volcanoes of Tertiary age. Denudation has laid bare the cen- 

 tral cores or necks of the old vents, surrounded by altered sedi- 

 mentary rocks through which the conduits were made. Radial 

 dikes traverse the sedimentary foundation, and accumulations of 

 scoria and various fragmentary volcanic rocks and lava flows 

 form the outer part of the mountain region and also a portion of 

 the central area. The accompanying figure represents a dia- 

 grammatic cross-section through the range, showing the stock 

 of granular rock exposed on Beaver Creek and the relations 

 of the effusive rocks to the basal platform of Cretaceous 

 shales. The geology is interesting, not only from these fea- 

 tures, which have their counterpart in the neighboring High- 

 wood Mountains, but also on account of the character of the 

 eruptive rocks and the bearing of their occurrence upon the 

 broad problems of petrology. The mountains have not been 

 glaciated, although the confluent terminal moraines of the two 

 great continental ice sheets skirt the flanks on all but the 

 southern sides. 



Cretaceous Massive Tuffs 



Strata. Igneous Rock, and flows. 



FiG. 2. — North and South section of Bearpaw Mountains through Bearpaw Peak 

 and Beaver Creek Core. 



The sedimentary rocks are Cretaceous, and in part at least 

 of Laramie age. They consist of a thickness of 2,000 to 3,000 

 feet of dark-colored shales, with interbedded sandstones and 



