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



miles along the foot of the mountains, as described by G. K. Gilbert.* 

 North of Beulah for several miles the " Dakota " sandstone lies directly 

 on the Fountain formation, but probably the Morrison beds formerly 

 covered the region and were removed by pre-Dakota erosion, a very 

 unusual relation. The Morrison beds south of Beulah, according to Mr 

 Gilbert, consist chiefly of red shale with a few layers of hard, red sand- 

 stone beds about 70 feet thick. Thej are faulted against the gneiss and 

 also overlap onto that rock for a portion of their course. The formation 

 appears again for 5 miles at the south end of the Greenhorn mountains, 

 lying partly on gneiss and partly on the Red beds. Its thickness here, 

 according to Hilis,f is 270 feet, the lower portion consisting of about 60 

 feet of soft, white sandstone, conglomeratic at base. The middle portion 

 is a series of pinkish and greenish, massive clays, and the upper beds are 

 variegated shales and clays, alternating with bands of fine grained lime- 

 stone, often containing vermilion colored cherts. Hills states that there 

 is considerable doubt as to the true position of the formation in the time 

 scale and assigns it to the Jurassic provisionally. 



"DAKOTA" SANDSTONE 



Under this heading there will be described the entire sandstone series 

 overlying the Morrison formation, a series which always has been known 

 as the u Dakota sandstone." It generally consists of two bodies of sand- 

 stone, each a hundred feet or more in thickness, separated by a deposit 

 of clay or shale 10 to 15 feet thick. The clay and the top sandstone have 

 yielded abundant plant remains of the Dakota flora (upper Cretaceous), 

 but there is less conclusive paleontologic evidence as to the age of the 

 basal sandstone series. The tripartite succession strongly suggests the 

 Dakota sandstone, Fuson clay, and Lakota sandstone of the Black hills. 



The " Dakota " sandstone is remarkably uniform in character through- 

 out eastern Colorado. The rocks are mostly hard and massive, giving 

 rise to a well marked hogback range along the foothills and plateaus, and 

 to steep walled canyons in the southeastern part of the state. The pre- 

 dominating color is light buff. Cross-bedding is almost general and con- 

 glomeratic streaks are frequent, especially at or near the base of the 

 lower sandstone. The contact with Morrison beds is abrupt and often 

 presents evidence of unconformity by erosion, but no more than is gen- 

 erally found wherever a coarse sand has been deposited on clay. There 

 is often a very rapid change to Benton deposits, but in most areas there 



*G. K. Gilbert: Description of the Pueblo district. Geologic Atlas U. S., folio 36, U. S. Geol. 

 Survey, 1897. 



f R. 0. Hills: Description of the Walsenburg district. Geologic Atlas U. S., folio 68, U. S. Geol. 

 Survey, 1900. 



