270 SMITHSOXIAX MISCELLANEOUS COLLECTIOXS VOL. 64 



jNIount Helena was formed of Cambrian rocks ; also that the upper 

 boundary of the Cambrian southwest, south, and southeast of Helena 

 would have to be placed at a higher horizon. This, however, does not 

 affect the map for the purpose of the present paper. 



CAMBRIAN SECTION 15 TO 20 MILES NORTHEAST OF HELENA, 



MONTANA, ALONG BEAVER CREEK, ON THE EAST 



SIDE OF THE MISSOURI RIVER 



Beaver Creek rises on the slopes of the north end of the Big Belt 

 Mountains and flows westward to where it empties into the Missouri 

 River, The formations are finely exposed along this canyon. Be- 

 neath the Flathead quartzite is a considerable thickness of siliceous, 

 slaty, dark shales of the Grayson formation of the Belt terrane that 

 strike north 42° west (magnetic), and dip 30° southwest 48° west. 

 These shales are overlain by the basal beds of the Flathead quartzite 

 which strike north 58° west (magnetic), dip 30° south 32° west. 

 This dip increases to 40° and gradually to 75° near the top of the 

 ridge on the north, and then returns to 35°. The strike at the top 'of 

 the ridge is north 50° west (magnetic). The section is given in my 

 field notes as from the bottom up as follows : 



ALGONK1.A.N 



Siliceous, slaty, dark shales of the Grayson formation, Belt 

 terrane. 

 Cambrian 



Flathead Sandstone 



Gray, massive-bedded, quartzitic sandstone, with a few con- 

 glomerate layers formed of small quartz pebbles. 



At 225 feet from base thinner-bedded quartz sandstones occur 

 and again at 355 feet a band of thinner beds comes in. At 640 feet 

 the massive beds of quartz sandstone give way to shaly sands and 



shales 640 feet. 



WoLSEY Shales 



Thin-bedded and sandy shales with irregular, thin-bedded, shaly 

 limestone carrying Middle Cambrian fossils at 180-200 feet. 

 Purple and green argillaceous shales appear at about 600 feet from 

 the base 695 feet. 



Intrusive layers of eruptive rock occur from above 100 feet at 

 various horizons as interstratified thin sheets that add about 120- 

 150 feet to the total thickness. Fragments of shales are included 

 in the eruptive layers on the north side of Beaver Creek where the 

 eruptive follows the parting of the shale on the lines of bedding 

 for long distances ; in places it cuts across the beds and drops a few 

 feet to another parting or disappears altogether. 



