NIOBRARA FORMATION. 67 



readily permit its division into three members, of which the lowesl is lime- 

 stone, having an average thickness of 50 feet; the middle is a succession 

 of gray marly clays of various shades; the upper, a series of yellow or buff 

 shales, 240 feet in thickness, containing several impure limestone bands. 



Lower division. Tlie limestone I it' tllC lower division, fro 1 11 its pr< )1 1 i i Ul ■} 1 1 



characteristics, forms an excellent datum-level in the study <>t' the strati- 

 graphic and structural geologv of the foothills along the Colorado Range. 

 These characteristics are, the bluish -gray, light-gray to clouded white 

 color; the even, fine-grained texture; the superior hardness by which it 

 effectively resists atmospheric influences and becomes persistent in outcrop; 

 the freedom from arenaceous and clayey matter; the conchoidal fracture; 

 tlie evenness of the bedding piano: and the fossil contents. The thickness 

 of this member is about ;">() feet, but the above characteristics are especially 

 applicable to the lower 30 feet, which is also a portion of particular eco- 

 nomic value. The upper layers are usually much thinner, and graduate 

 through shales into the overlying group of marls constituting the middle 

 member of the formation. Occasionally the limestone becomes shaly from 

 base to summit, when the prominence of its outcrop is greatly diminished. 

 The presence of bituminous matter, though recognizable, is not so marked 

 in the limestones of the Niobrara as in those of the Benton. 



Following is an analysis of a type specimen of the limestone, l>v Mr. 

 L. (jr. Eakins, which shows it to he dolomite: 



Analysis »f limestone. 



Per c< hi 



Insoluble 12. 01 



CaO 27.411 



MgO 18.03 



MnO 20 



A1;0 : 54 



F.-.O; 11 



PsOs 029 



H 2 01 



CO ; 11. lu 



ioo. ii:i 

 Local variations from the above composition may easily he seen under 

 the lens alone. These consist of differences in the amount of silica present 

 in the form of sand; in varying proportions of iron, as instanced by the 



