Weed cmd Pirsson — Bearpaw Mountains of Montana. 189 



The rock types represented include syenite porphyry, " leu- 

 cite" basalt (this rock also occurring as extrusive flows and 

 breccias), tinguaites in considerable variety, and micaceous 

 rocks that are probably to be classed as minettes. The first 

 two types do not need especial mention. They were seen on 

 Beaver Creek and in various other parts of the mountains. 



The minette-like dikes were observed at many of the points 

 visited; they occur in abundance about the Wind Butte stock 

 and on Beaver Creek. 



Tinguaite Dikes. 



The tinguaites form the most striking and characteristic 

 rock of the mountains. The term is used for those dense 

 rocks, whether porphyritic or not, which are composed chiefly 

 of alkali feldspars, and carry such large amounts of segirite in 

 the fine-grained groundmass that they have a pronounced 

 greenish color, and megascopic habit that furnishes a useful 

 characteristic for discrimination."^ These rocks form the 

 most conspicuous feature of the drift both within and about 

 the mountains, their bright green color, large white feldspar 

 crystals, and the glassy polish which the surface assumes upon 

 weathering, making them appear in striking contrast to the 

 other rocks with their dark tones of color. These tinguaitic 

 rocks are not, however, of as frequent occurrence as the wide- 

 spread abundance of the drift would indicate, as the rock is 

 very tough and resistant, and resists disintegrating agencies 

 better than any other rock type of the mountains. 



East of the Wind Creek stock the high ridges forming the 

 divide between the People's Creek basin and the headwaters 

 of Bean and Snake creeks show a swarm of tinguaite dikes, 

 forming walls which are conspicuous features of the grassy 

 ridges and extending eastward to the main road. 



Tinguaite porphyry . — On the high ridge running east from 

 Wind Butte, some twelve or more nearly parallel dikes of this 

 rock were observed. The rock seldom forms continuous walls, 

 though it is very hard, tough and resistant, rings sonorously 

 under the hammer, and is broken with great difficulty. It is 

 jointed, and breaks in large cubical blocks. The shales and 

 sandstone beds are highly altered in the space between the 

 dikes. A decomposed brownish feldspathic rock appears to be 

 a frequent accompaniment. The dikes vary from a few feet 

 to 12 feet in width. At the contact the rock is dense and 

 without the large porphyritic crystals which are so conspicu- 

 ous in the main mass of the dike, — a most important feature, as 



* This Journal, vol, 1, 1895, p. 399. 



Am. Jour. Sci. — Fourth Series, Vol. II, No. 9. — September, 1896. 

 14 



