Weed and Pirsson — Bearjpaw Mountains, Montana. 283 



Art. XXXIY. — The Bearpaw Mountains, Montana* First 

 Paper ; by Walter Harvey Weed and Louis Y. 

 Pirsson. 



Introduction. — The present paper gives an account of the 

 prominent features of a mountain group in northern Montana. 

 It consists of a cluster of dissected volcanoes whose breccias, 

 tuffs, and flows compose a large part of the area, especially the 

 marginal region ; while cores of massive rock and associated 

 dikes and bosses are found cutting soft Cretaceous shales in the 

 central parts of the mountainous tract. The rocks present 

 interesting examples of the differentiation of an alkaline 

 magma, and consist of various types of the syenite family 

 of igneous rocks, together with highly differentiated basic 

 forms that accompany them. The only mention of the geology 

 of the region that we have been able to discover is a brief note 

 by Dr. Waldemar Lindgren,-f who examined some specimens 

 of rocks, mostly altered, collected from the southern foot hills 

 by Dr. C. A. White in 1882. 



The Bearpaw Mountains form one of the isolated mountain 

 areas that rise abruptly above the grassy plains of central Mon- 

 tana. They consist of a group of peaks and are not a true moun- 

 tain range, and lie between the meridians of 109° and 110° 

 west, and parallels of 48° and 48° 50' north. They are situated 

 far to the east of the Rocky Mountain front, and north of the 

 swift flowing Missouri River. The region is readily accessible 

 by a mail stage from Chinook, a town of some size situated 

 upon the main trans-continental line of the Great Northern 

 Railway. The valley lands of the eastern part of the moun- 

 tains are settled, and there are three post offices in this portion 

 of the region. The western and most highly accidented part 

 lies within the limits of the Fort Assiniboine military reserva- 

 tion. 



Topography. — In scenery there is a strong contrast between 

 the billowy, rolling surface of the plains covered with glacial 

 drift and the abrupt slopes and crag crowned summits of the 

 mountainous tract. Several isolated buttes rising above the 

 plain far out from the mountains are genetically connected with 

 them, but will not be considered here. 



Seen from the level plains, the mountains present a skyline 

 of more or less conical peaks, in part connected with one 

 another, in part as separate elevations of nearly equal height, 



* By permission of the Director of the U. S. Geological Survey. The present 

 paper is based upon notes and collections made by one of the authors (W. H. W.) 

 during a brief visit to the region in October, 1895. 



f This Journal, vol. xlv, April, 1893, p. 281. 



Am. Jour. Sol— Fourth Series, Yol. I, No. 4. — April, 1896. 

 19 



