Douglass: Geology of Southwestern Montana. 411 



ALGONKIAN. 



Not greatly Metamorphosed. 



In portions of middle western Montana there are thick and exten- 

 sive exposures of sandstones, slates, and shales or argillites, with some 

 conglomerate and limestone, which in some places, at least, are known 

 to lie beneath Cambrian rocks. 



A portion of the beds in the vicinity of their eastern outcrops have 

 been mapped and described in the atlas sheets and other publications 

 of the U. S. Geological Survey, which give the results of research 

 here. They have been observed in the Big Belt Mountains, Bridger 

 Mountains, the Horseshoe Hills (west of the northern portion ot the 

 Bridger Mountains between this range and the Missouri River), and in 

 the mountains northeast of Helena. 



In the region west of the Rocky Mountains similar rocks cover a 

 large area. Between the meridian of 113° and the Bitter Root 

 Mountains, and between the parallels or 46° and 47° they are more 

 common than any other rocks. In the region around Missoula there 

 is little other rock older than the Tertiary. The Hell Gate Cailon 

 from Missoula to Beramouth and the lower portion of the Big Black - 

 foot Valley are in quartzytes and slates that are probably Algonkian. 

 In the western exposures they appear to be on the whole more arena- 

 ceous. 



All of these beds are distinctly stratified, sometimes very thinly 

 laminated and beautifully banded. The prevailing color is reddish, 

 or brown, of the color of iron rust, with bands and series of layers 

 which are green. In the Smith River Valley there are cream-colored 

 shales or slates which break into thin flakes. At a distance the color 

 and the general relief make them appear like the Miocene deposits 

 which occur in the same locality. 



These beds do not, as a rule, have a very ancient appearance. They 

 are not highly metamorphosed. The sandstones have been changed 

 to quartzytes, but of course that is common in deposits of all ages. 

 They are seldom greatly disturbed. They usually dip at gentle angles 

 and there is not that almost inextricable confusion in which the 

 Palaeozoics, especially the Devonian and Carboniferous, have become 

 entangled, in so many localities. In places where the Palaeozoic 

 limestones are near or in contact with the Algonkian, the former is 

 often greatly distorted and disarranged, sometimes forming rugged, 



