UPPER CRETACEOUS. 663 



at 375 feet, but it is probable that he included in the Dakota some Comanche strata 

 which had not at that time been recognized in the region. Hills says:^™ 



The lower two-thirds of the Dakota consists of sandstones, with fine conglomerates, 

 imperfectly stratified or cross-bedded, and the heavy layers which make up this part of the 

 formation are separated from one another by thin bands of finer, shaly material. The upper 

 one-third also consists of sandstone layers parted from one another by thin bands of shale, 

 but the individual beds are not so thick and the shale partings are more numerous. These two 

 portions are separated from each other by a pronunent bed of hard shale — fire clay — the position 

 of which, in cliBF exposures, is often indicated by a narrow shelf or terrace immediately below it. 



The color of the upper sandstone is generally grayish white, the lower somewhat darker, 

 with yellowish and brownish weathered surfaces. The pebbles of the coarser layers are quartz- 

 ite, quartz, and chert. The finer-grained layers are made up of quartz grains, among which 

 white, kaolin-like specks are included. The lower sandstone is of an open, porous texture and 

 more loosely aggregated than that lying above the fire-clay stratum. But the texture of any 

 particular layer varies from place to place, and the same is true of the thickness of the indi- 

 vidual beds, so that the only constant features are the fine-grained, compact sandstone above 

 and the coarser, porous sandstone below the persistent bed of fire clay separating them. 



The Dakota is followed in succession by the Graneros shale, the Greenhorn 

 limestone, and the Carlile shale, which constitute the Benton group. Of these for- 

 mations Hills says : 



The Graneros formation, which marks the beginning of the marine conditions following 

 the subsidence that terminated the Dakota epoch, consists of dark-gray clay shale, from 200 

 to 210 feet in thickness, darker near the center than elsewhere, resting on the Dakota sandstone 

 and graduating rather abruptly into it. Large limestone concretions are not uncommon in the 

 upper half but are not a distinguishing feature, as similar concretions are met with in the other 

 shale formations of the district. At a distance of about 30 feet above the base there is a layer, 

 from 1 to 2 feet thick, of hard concretionary limestone, weathering an orange tint, which is 

 noticeable and characteristic. 



The Greenhorn formation is made up of layers of dove-colored limestone, usually less than 

 12 inches thick, separated from one another by somewhat thicker layers of shaly material. It 

 graduates into the Graneros shale below and the Carlile shale above. FossU shells are abundant 

 in the limestone layers, especially the flat, oval, concentrically ridged Inoceramus lahiaius. 

 The coiled ammonite Prionocyclus is sometimes present. The thickness varies from place to 

 place, owing to the thickening or thinning of the shaly layers. At the same time the gradua- 

 tion into the Graneros and Carlile formations is more abrupt in some places than in others, thus 

 rendering it doubtful at times where to draw the line. The maximum thickness occurs in the 

 northeastern part of the quadrangle, where it is often 60 feet. In the southeastern part the 

 exposed sections are not so complete and it is thought that in some places the thickness may 

 not exceed 30 feet. * * * 



The CarUle formation consists of about 180 feet of dark-gray shale, the middle portion the 

 darkest, overlain by from 10 to 15 feet of soft, shaly yellowish-gray sandstone, into which it 

 graduates through a varying thickness of more distinctively shaly material. A thin band of 

 purplish bituminous limestone containing large numbers of coiled ammonites is persistently 

 present capping the formation. Concretionary nodules several feet in diameter and seamed 

 with lime spar are rather common, especially in the upper half of the beds. 



Overljdng the Benion group is the Niobrara group, concerning which Hills 



says:*®'* 



The Timpas and Apishapa formations, both of which are characterized by the presence of 

 Hmestone strata, or of shale containing a considerable proportion of lime, constitute a Niobrara 



