148 GEOLOGY OF TUE BLACK UILLS. 



Jura. 



Feet. 



4. Sandstone, yellowish, sometimes quite red; forming the floor of the lower 



plateau in Figure 24 50 



3. Clay or marl, gray or greenish, with a pink streak near the middle; becoming 

 in the lower .'{ feet an impure slialy limestone, with many Jurassic fossils. 

 Immediately above the latter layer there is considerable Hinty matter 120 



Red Beds. 



2. Sandstone, deep red, argillaceous and very shaly, containing a bed of gypsum 8 



1, Sandstone, deep red, soft and sludy, with several seams of gypsum in its 



upper 4 or 5 feet to stream . . 171 



The red clays near the junction with the fossiliferous strata were 

 observed to have a browner color than below. On the opposite side of the 

 river the same fossiliferous strata were seen, and underneath them, below 

 60 feet of red cla}', is a stratum 20 feet thick made up of seams of gypsum 

 1 foot thick separated by bands of clay, while below it are the red sandy 

 clays of the section. Northward, down the river, the Red Beds retain the 

 characters, described In some places they become so firm as to form the 

 immediate banks of the river for considerable distances, rendering the cross- 

 ing a matter of difficulty. Where they finally sink beneath the river they 

 attain their depression by a gradual dip, and give place to the Jurassic 

 strata, which in turn shortly dip under the river and are replaced by the 

 lower Cretaceous series. 



One other outlying outcrop of the Red Beds remains to be mentioned. 

 It lies on the west base of Bear Butte, a few miles northeast of Crook City, 

 and what little is known of its character is detailed in the description of 

 Bear Butte in the section on the igneous rocks. 



'a' 



In conclusion, a brief notice may be given of the Red Beds or Trias 

 in other portions of the Rocky Mountain region, for the sake of showing 

 the more general relations of their development in the Black Hills. 



A series of red beds, provisionally referred by most authorities to the 

 Trias, is found on the eastern slope of the Rocky Mountains almost con- 

 tinuously from British America to Mexico, and in Indian Territory, Texas, 

 and New Mexico it covers a large area of country. It occupies the same 

 geological position as the series already described in the Black Hills, and is 



