890 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OP MONTANA. 



Page 



Plagioclase .....' 412 



Nepheline 412 



Cancrinite 413 



Sodalite 413 



Natrolite 413 



Chemical composition 413 



Structure and classification 414 



Name shonkinite 415 



White rock or sodahte-syenite 416 



General petrology of Square butte 417 



Summary 422 



Part I. — Geology of the Highwood Mountains. 



SITUATION. 



The group of mountaiDS of which a brief account is given in this paper 

 form one of the detached mountain groups lying east of the Rocky Moun- 

 tain Cordilleras and rising abruptly from the level plains of central Mon- 

 tana. A glance at the map of the state shows the Missouri river, born of 

 the Jefferson, Madison and Gallatin, in the mountain-encircled Gallatin 

 valley, cutting its way in a general northward course through the outer 

 ranges of the Rocky mountains until it debouches on the great open 

 plains of the northwest. From here it turns in a rude arc, flowing east- 

 ward over the cataracts which have given rise and name to the city of 

 Great Falls, and past Fort Benton, the head of steamboat navigation. In 

 this arc, between the Missouri and the Little Belts, a front range of the 

 Rocky mountains, the Highwood mountains are enclosed, and they are 

 about 30 miles east of Great Falls and 25 miles south of Fort Benton. 

 From each of these cities the peaks of the range form the most conspic- 

 uous feature of the landscape and break the monotonous level of the 

 plains. The meridian of 110° 30' west longitude and parallel of 44° 30' 

 north latitude pass through their center. 



It is not proposed in this brief paper to present more than the salient 

 features of the mountains as revealed by the field-work of the past summer, 

 when the region was studied while mapping the geology of the Fort Ben- 

 ton sheet of the geologic atlas of the United States Geological Survey. 



Previous knowledge of the geology of this mountain group is confined 

 to a few notes published by W. M. Davis and W. Lindgren, who were at 

 that time members of the Northern Transcontinental Survey.* These 

 observers recognized the fact that the mountains were largely formed of 

 Cretaceous strata penetrated by intrusions of igneous rocks. Later papers 



* Tenth Census of the United States, vol, xv, p. 734. 



