432 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



240 feet, and east of Lyons 125 feet. It consists mainly of clay of gray 

 color, slightly darker below than above, with more or less sand, especially 

 to the westward along the Rocky Mountain front, where its upper mem- 

 ber is usually a gray sandstone 6 to 15 feet thick. East of Pueblo, in 

 the Arkansas valley, the sandstone is replaced by a purple limestone 2 to 

 3 feet thick, containing fossils of which the most characteristic is the 

 Prionocy das Wyoming ensis, and about 125 feet lower down there is usualty 

 a thin bed of impure limestone containing fossils. Near the top of the 

 formation the shales contain numerous lens-shaped lime concretions, in 

 greater part from 2 to 4 feet in diameter, containing Prionotropis woolgari. 



NIOBRARA FORMATION 



The Niobrara outcrop extends from north to south across central Colo- 

 rado, a short distance east of the foothills. Tt has extensive surface 

 outcrops along the Arkansas valley. The upper members consist mostly 

 of impure chalk and calcareous shale, while at the base of the formation 

 there are beds of moderately pure hard limestone or dolomite. The 

 thickness in the Pueblo region and for some distance down the Arkansas 

 valley is 700 feet ; but west of Denver and in Kansas it is only about 400 

 feet, a diminution in thickness to the north and to the east which appears 

 to be gradual. The calcareous shale of the upper portion of the forma- 

 tion has a thickness of about 500 feet in the Pueblo region and half as 

 much in the Denver region. The beds are gray in color when fresh, but 

 on weathering develop a distinctive bright yellow tint. Occasional thin 

 sandy beds occur and many thin masses of limestone filled with Ostrea 

 congesta. In the Arkansas valley the formation contains concretions 

 which are often of considerable size. In this valley there is an inter- 

 mediate member of light gray shales and marls containing a large 

 amount of lime and occasional thin beds of limestone with Ostrea con- 

 gesta. This member is from 100 to 125 feet thick, and it grades down- 

 ward into a persistent bed of hard limestone, which is the westward 

 continuation of the Fort Hays limestone of Kansas. The thickness of 

 the limestone averages about 50 feet, consisting of beds in greater part 

 from 8 inches to a foot thick, separated by thin layers of gray calcareous 

 shale. 



The rock is of light gray color, weathering nearly white on some 

 of the surfaces, and is close textured, fine grained, and moderately re- 

 sistant. The characteristic fossil of the lower limestone is the Inoceramus 

 deformis, usually associated with a greater or less amount of Ostrea con- 

 gesta. Northward in Colorado the formation usually presents three 

 beds of limestone, the lower one the thickest, separated by calcareous 



