RESUME BENTON GROUP 445 



top, while concretions usually occur not far below this sandy horizon. 

 The Greenhorn limestone is characterized by great colonies of Inoceramus 

 labiatus, a species which rarely is found at all in other horizons, while 

 in the upper portion of the Carlile occurs Prionotropis woolgari, which 

 appears to be restricted to that horizon and to characterize it throughout 

 the region and even in the Bighorn basin. Throughout eastern Wyo- 

 ming and the Black Hills region there is in the middle part of the 

 Graneros shales, not far above the local sandstone horizon, a series of 

 hard gray shales and thin bedded sandstones filled with fish scales, 

 which weather to light gray color and from their hardness give rise to 

 a ridge or cliff. These have been termed the Mowry beds, and they are 

 conspicuous along both sides of the Bighorn, all around the Black hills, 

 and along the Laramie front to the Colorado line. 



In plate 36 sections are given showing the principal stratigraphic fea- 

 tures of the Benton group in different districts. The variations in thick- 

 ness are very striking, especially between 400 feet in Kansas and 1,300 

 feet, which is the average thickness in the Black Hills region. The 

 salient features in the Kansas section are the Greenhorn limestones, com- 

 prising several limy layers having in all a thickness of 60 feet, of which 

 40 feet near the middle are characterized by large numbers of the typical 

 Inoceramus labiatus. The Kansas geologists have included about 50 feet 

 of the basal shales of the formation in the Dakota, but the reasons for 

 this inclusion are not convincing, and I should be inclined to regard 

 the saliferous and gypsiferous shales as comprising the lower portion of 

 the Graneros. The three main subdivisions are readily distinguishable 

 throughout eastern Colorado, where the formation gradually increases in 

 thickness, mainly by the expansion of lower beds. In this region also 

 there first appears, near the base of the Graneros formation, a bed of 

 sandstone which varies greatly in thickness, but often gives rise to a 

 conspicuous subordinate hogback ridge lying east of the main " Dakota " 

 hogback, or on its slope. In southeastern Wyoming a further increase 

 of thickness is exhibited. Here the Greenhorn limestone finally be- 

 comes thin and discontinuous approaching the end of the Laramie 

 range and it is not recognizable at all in the sections along the Bighorn 

 uplift. It is, however, a conspicuous feature in the slopes adjoining the 

 Black hills, extending entirely around that uplift and often attaining a 

 thickness of 50 feet. The sandstone in the lower portion-of the Graneros 

 appears to extend almost continuously through southeastern Wyoming 

 and it appears at intervals around the Black hills, notably at Newcastle, 

 Hermosa, and at the northern end of the uplift, but it does not appear 

 in the Bighorns unless possibly at one locality. 



The Mowry beds, consisting of hard shales and thin sandstones with 



