THE COLORADO FORMATION 89 



onocyclus Wyomingefisis, Placenticeras placenta, Inoceramns undabun- 

 dus and /. tcmiirostratus. 



In the Kansan area the Blue Hill shales have a thickness of 

 100 feet. The upper Carlile shales form their stratigraphical 

 equivalent in the Colorado area. The septaria zone occurs in 

 relatively the same position in the Carlile shales, and presents 

 approximately the same lithological characteristics. The total 

 thickness of the Carlile shales is from 175 to 200 feet, including 

 the upper and lower beds, the equivalents of the Blue Hill and 

 Ostrea beds. In the Black Hills area a bed of shales thirty to 

 forty feet in thickness, bearing calcareous concretions of the 

 cone-in-cone structure is the stratigraphical equivalent of the 

 Blue Hill shales. The Blue Hill shales as well as the Ostrea 

 shales have no equivalent in the Iowa-Nebraska and eastern 

 Dakota areas. 



THE NIOBRARA 



The division line between the Benton and the Niobrara in 

 the Kansan area at least is not an arbitrary one. Lithologically 

 it marks a change from dark argillaceous shale to comparatively 

 pure chalk of remarkable whiteness, scarcely compact enough to 

 deserve the name limestone. The change is one of abruptness, 

 there is no transition zone. The shale does not appear again. 

 A massive stratum of limestone rests upon the dark shales, and 

 there is no intermediate layer, part chalk and part shale. Palae- 

 ontologically the change is marked by the ushering in of an 

 almost entirely new fauna. 



Two principal divisions of the Niobrara are recognized in the 

 Kansan area. These are the lower or Fort Hays limestone and 

 the upper or Pteranodon beds. The division may be said to 

 rest on both lithological and palaeontological grounds. The 

 Pteranodon beds are further subdivided into the Rudistes and 

 Hesperonis beds. This division is made on purely palaeontologi- 

 cal evidences. 



The Fort Hays limestone. — The Fort Hays limestone is in 

 massive lavers of from two to four feet in thickness, and the 



