CLOVERLY AND BENTON FORMATIONS 399 



of darker gray color and with a thickness of from 30 to 40 feet. Near 

 the base of the sandstones there occur some very thin deposits of coal 

 or coaly shale which sometimes contain remains of numerous flattened 

 pine needles, as in the coal deposits of the lower beds of the Lakota of 

 the Black hills. 



BENTON FORMA TION 



The lower portion of the great mass of shales underlying the plains 

 along the foot of the Bighorn mountains belongs in the Benton forma- 

 tion. It has an average thickness of about 1,300 feet. The subdivisions 

 so obvious in the Black hills and in Colorado are not recognized, mainly 

 owing to the absence of the Greenhorn limestone. However, the char- 

 acteristic series of hard shales which weather to a light gray color are 

 present, here nearly a thousand feet above the base of the formation, 

 which indicates a great expansion of the lower third of the Graneros 

 shales. 



The basal member of the Benton consists of dark gray shales, in part 

 sandy and of rusty brown color, with occasional thin beds of brown 

 sandstone. Locally the sandstone expands into a bed of moderate 

 thickness. It is possible that this portion of the formation represents 

 the Dakota sandstone of other regions, but there is no direct evidence, 

 and even if the few indistinct plant remains which it contains should 

 prove to belong to the Dakota flora, that would be no more than we 

 should expect in any shallow water deposits at the beginning of Benton 

 times. There occurs in the upper part of this lower series, all along 

 both sides of the Bighorn uplift, a zone of shales carrying round concre- 

 tions varying from three-quarters of an inch to 2 inches in diameter 

 in greater part, with radiated, crystalline structure and dark gray color. 

 They consist mostly of phosphate of lime and appear to have the struct- 

 ure of marcasite and to be a replacement of that mineral. 



Next above this lower member there are several hundred feet of dark 

 shales, mostly fissile, which contain thin beds of sandstone and iron 

 and lime concretions of t}^pical lower Graneros character. These are 

 overlain by a very characteristic series, about 150 feet thick, of hard, 

 lighter gray shales and thin bedded sandstones which weather to a light 

 gra}^ color and form bare ridges of considerable prominence, notably 

 Columbus peak, southwest of Parkman. Most of its beds contain large 

 numbers of fish scales and occasional fish teeth and bones. By some 

 observers this member was supposed to be Niobrara, but it lies below 

 beds containing distinctive Benton fossils, and in other portions of east- 

 ern Wyoming it is exposed, having relations which indicate that its 

 position is below the middle of the Graneros division of the Benton 



