204 C. D. WAfX'OTT rRE-CAMBRIAN FOSSILIFEROUS FORMATIONS 



by Mr Leon S. Griswold, who was assisting Mr Weed in mapi)ing the 

 areal geology about Helena. 



The results of my investigations were the discover}^ of a great strati- 

 graphic unconformity between the Cambrian and the Belt formations ; 

 that the Belt terrane was divisible into several formations, and that 

 fossils occurred in the Greyson shales nearly 7,000 feet beneath the high- 

 est beds of the Belt terrane. 



The rocks of the Belt terrane are widely distributed in central Mon- 

 tana, the best sections being exposed in the Big Belt and Little Belt 

 mountains, in Meagher and Broadwater counties. They extend to the 

 -north in Cascade,. to the west in Lewis and Clarke and Jefferson counties, 

 and to the south in Gallatin, Park, and Sweetwater counties, in all cover- 

 ing a known area of more than 6,000 square miles. Throughout this 

 area the Cambrian rocks rest unconformably on different members com- 

 posing the Belt terrane. The general outline of the areal distribution 

 of the Belt rocks is shown on the accompanying sketch map, figure 1, 

 prepared from data furnished by ]Mr \V. H. Weed. 



The principal members of the Belt terrane are as follows : 



Marsh shales 300 feet. 



Helena limestone 2,400 " 



Empire shales 600 " 



Spokane shales 1,500 ' ' 



Greyson shales 3,000 ' ' 



Newland limestone 2,000 " 



Chamberlain shales 1,500 ' ' 



Neihart quartzite and sandstone 700 " 



12,000 feet. 



Neihart quarlzlte and sandstone. — In this formation are included the 

 reddish, coarse sandstones, with interbedded dark greenish layers of fine 

 grained sandstone and shale, beneath the Chamberlain shales. The 

 lower 400 feet of the formation is a massive, sometimes cross-bedded 

 quartzite, which, in some of its members, where unaltered, is a compact, 

 hard sandstone. The prevailing color is pinkish gray on the freshl}^ 

 exposed surface, with dark and iron-stained weathered surface. Occa- 

 sional layers of a fine conglomerate occur in some portions near the 

 contact with the gneiss. 



This formation was named by Mr \\'eed from its occurrence at the 

 type locality on Neihart mountain, where the quartzite and sandstones 

 are in contact with the gneiss, and di{) to the southeast, crossing Belt 

 creek and passing beneath the superjacent formation in the canyons of 

 Sawmill and Chamberlain creeks. 



The thickness of the formation, as measured by Mr Weed, is 700 feet. 



