444 N. H. DARTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



Fuson, and Dakota is found, but from the Laramie river southward there 

 appears to be only one sandstone, which, so far as its character goes, 

 might belong to either one sandstone member or the other. In Colorado 

 there are usually two sandstones separated by a bed of fireclay, which 

 strongly suggests the Lakota-Fuson-Dakota succession, but it has been 

 supposed that all the rocks are of Dakota age. The intercalated fire- 

 clay series extends far southward through Colorado and eastward in the 

 exposures of the Purgatoire and other canyons. 



Throughout Colorado and eastern Wyoming, as in the region north- 

 ward, the sandstones are underlain by the Morrison formation, excepting 

 at a very few localities, and although there is general unconformity, it 

 is difficult to believe that this represents Lakota and Fuson time. The 

 Morrison materials are soft, and if they were exposed to erosion for such 

 a long period, they undoubtedly would have suffered deep and wide- 

 spread degradation. It is unlikely also that the Lakota and Fuson could 

 have been deposited in regular order, and then been so completely and 

 evenly removed as to leave the present stratigraphic relations of the 

 Morrison to its overlying sandstones. A discovery very significant in 

 this connection has been made by J. B. Hatcher, that dinosaur remains 

 of Morrison type occur in the lower members of the overlying sandstone 

 beds north of Canyon City, which indicate that there could have been 

 no great time break at this horizon. 



For these reasons it is probable that the Lakota-Fuson-Dakota series 

 extends southward in Wyoming and Colorado, and possibly careful 

 search in the medial fireclays and lower sandstones will yield lower 

 Cretaceous plants. 



BENTON GROUP 



The rocks of this group are the most widespread and constant in 

 characteristics of all the sedimentary deposits of the Central Plains 

 region. The general feature of a thick succession of shales overlying 

 the Dakota sandstone is the salient one, but widespread subdivisions or 

 horizons of variation have also been recognized. Its thickness is variable, 

 ranging from about 400 feet in the southeast to 1,600 feet in the Black 

 hills. In nearly all the half million square miles under consideration . 

 the group comprises three members — a basal dark shale series known 

 as the Graneros shales, a medial limestone known as the Greenhorn 

 limestone, and an upper shale series with sandy layers, known as the 

 Carlile formation. Toward its base the Graneros shale includes a hori- 

 zon marked by thin but extensive layers of sandstone. The Greenhorn 

 limestone always presents alternations of slabby limestone and shales, 

 and the Carlile formation generally has a sandstone bed at or near its 



