274 



WINTHROP P. HAYNES 



The extensive exposure of the Belt Series north of Three Forks, 

 which, so far as the writer was able to ascertain, has not been 

 described before, consists of two fairly distinct formations which 

 are considered to be equivalent to the Spokane and Empire forma- 

 tions of the Belt Series. 



Spokane formation. — -In the vicinity of the double horseshoe 

 bend of the Missouri River there is a fine section through the 



Fig. 3. — Domal structure in ridge west of the Missouri River 



Spokane formation. The formation at this place consists of a thick 

 series of well-stratified red and green slates with frequent layers of 

 ripple-marked and mud-cracked sandstone. The finer beds are 

 mostly very hard and siliceous and may be called argillites or even 

 metargillites. At several places in the section distinct folds are 

 visible, and also some faults. The minimum thickness of the 

 formation in this section is 1,650 feet, but the average thickness 

 is probably considerably greater than these figures. 



Empire shale. — -This formation, which overlies the Spokane 

 formation, is exposed in a long strip west of the Missouri River, 



