392 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



which everywhere score the higher elevations. This abundance of 

 moisture favors agriculture, and the upland valleys and mountain foot- 

 slopes are occupied by ranches. The cereals and a great variety of vege- 

 tables not commonly raised in the State are grown and reach maturity 

 despite the short season. The surrounding country is very generally 

 given over to raising cattle or sheep. 



GENERAL GEOLOGIC STRUCTURE. 



The Highwood mountains consist of the denuded remains of a group 

 of volcanoes whose rocks show extreme differentiation of a highly alka- 

 line igneous magma. They contain several volcanic cores or conduits, 

 now filled with massive granular rock. These are surrounded by tuffs 

 and volcanic breccias, with lava-flows, and great numbers of radiating 

 dikes. 



The main mass of the mountains consists of basaltic breccias, resting 

 upon Cretaceous sediments and the earlier acidic tuffs and breccias, 

 which form the foothills to the east and northwest. The metamorphosed 

 sedimentary rocks about the volcanic cores form a lesser but prominent 

 feature, as their resistance to erosion has left them as important peaks. 

 In approaching the range, the great number of dikes which stand up as 

 walls, extending often for miles across the level bench land, form a con- 

 spicuous and most interesting phenomenon. On the lower mountain 

 slopes they are even more numerous, and easily suggest that the moun- 

 tains themselves are of sedimentary rocks penetrated by dikes and igne- 

 ous masses. The radial disposition of the vast number of these dikes is 

 one of the most marked characteristics of the geologic structure. From 

 the summit of South peak they may be seen radiating outward, traceable 

 with a glass for miles, over the open grass-covered country below. Even 

 those composed of rocks which weather readily to the general level can be 

 traced by the grass and herbage on their outcrops, for the soft, sandy soil 

 produced by their decomposition is markedly different from the soils of the 

 Cretaceous rocks, and the difference is accentuated by the plant growth. 

 Often, moreover, these dikes have baked the adjacent rocks and both 

 edges of the contact are marked by narrow lines of hardened and gener- 

 ally bare outcrops. 



From the crest of South peak, owing to the above facts, the course of 

 the dikes can be traced over the open brown slopes as rough black walls 

 of projecting rock, vivid bands of green or by parallel bands of dark color 

 appearing like long lines of sepia drawn on a surface of brown paper.* 



The view from the summit of Palisade butte or of Alder peak is even 

 more remarkable. Here the dikes appear as massive stone walls, from 



*See also Davis, loc. cit. 



