10 C. D. WALCOTT ALGONKIAN FORMATIONS OF MONTANA 



stones of the Cambrian Flathead formation form the northwest side and 

 the reddish brown sandstones and shales of the Algonkian Belt terrane 

 the southeastern side, the Algonkian lying nnconf ormably below the 

 Cambrian. 



Algonkian, Belt terrane. — The Algonkian at the Lewis and Clark pass 

 comprises the Marsh, the Helena, and the Empire formations, as follows : 



Marsh formation 



Feet Feet 



1. Reddish brown, arenaceous shales and sandstones, similar 



to the Marsh shales of the Belt terrane 790 



2. Gray, hard, arenaceous shales and sandstones, with alter- 



nating bands of reddish brown, thin bedded and shaly 

 sandstones, 20 to 40 feet in thickness 225 



Helena formation 



Thin bedded, gray, siliceous limestone, with numerous are- 

 naceous layers in the upper 25 feet. The central and 

 lower portions have interbedded, bluish gray limestone 

 with occasional layers of interformational conglom- 

 erate, formed of thin, shaly limestones. The siliceous, 

 hard layers weather a yellowish buff color. 285 



Empire formation 

 Gray, arenaceous shales and thin bedded sandstones. A bed 

 of Cryptozoan limestone two feet thick occurs near the 

 top and a similar bed 360 feet below 1,210 



Kesume of Algonkian, Belt terrane 



Marsh formation: 



1. Reddish brown, arenaceous shales and sandstones 790 



2. Gray, arenaceous shales and sandstones 225 



Helena formation: 

 Gray siliceous limestones 285 



Empire formation : 

 Gray, arenaceous shales and sandstones 1,210 



Total section 2,510 



SWAN RANGE SECTION 



Location of the section. — The section of the limestones and interbedded 

 siliceous strata of the Swan range, south of Holland peak, begins be- 

 neath the red sandstones of the Camp Creek series and extends westward, 

 over the crest of the range, nearly to its western base. 



