THE UPPER CRETACEOUS GROUPS. - 181 



same bitter salts are found also in the higher beds but in smaller quantity. 

 Selenite, in well defined and clear crystals, is very abundant and dots the 

 exposures of the formation with bright spots that glisten like jewels in the 

 sunlight. 



On Rapid Creek the Fort Benton cla} 7 s have a thickness of 150 or 200 

 feet, and dip eastward at about 20°. They rest upon the Dakota sand- 

 stone, and are covered in turn by a bed of sandstone 10 or 15 feet thick, 

 which forms a ridge in the topography. Beyond this ridge a furrowed and 

 rolling surface extends eastward to the Cheyenne, where No. 4 forms the 

 bank of the stream. On the east side of the river Nos. 4 and 5 are overlaid 

 by outliers of the White River Tertiary. 



At the pass of the foothills on French Creek the Dakota sandstone, 

 dipping eastward at an angle of 8° or 10°, is overlaid by the following 

 series : 



Feet. 



1. Clays of the Fort Benton, dark brown or black 150 



2. Sandstone, white, coarse; somewhat conglomeratic at base 40 



3. Clays, yellow and somewhat arenaceous ; with thin shaly limestone near the 



top ; containing Inoceramus 50 



Nos. 2 and 3 of this section probably belong to the Niobrara group, 

 the total thickness of which we could not readily estimate. Professor 

 Winchell, who enjoyed better opportunities than we at this locality, places 

 it at 150 feet In the same vicinity he assigns to the Fort Pierre group a 

 thickness of 250 feet. 



The cascade on Minne Katta Creek, which is a little over two miles 

 from the Cheyenne, is in the gap through which the creek passes the ridge of 

 Dakota sandstone. The sandstone has a dip to the south and east of about 

 30°. Above it are the Fort Benton clays about 150 feet in thickness, 

 which thence form the banks of the creek to the Cheyenne. In this short 

 space the dip changes so rapidly that at the river the strata lie very nearly 

 horizontal. Conspicuous hills near the mouth of the creek consist of shales 

 with some sandstones, and probably exhibit a portion of the Niobrara 

 group. 



