BENTON FORMATION. 65 



where the earliesi work in the Cretaceous rucks indicated a much more 

 distinct line of demarcation than thai now recognized. 



BENTON. 

 STRATIGRAPHY. 



This is essentially a formation of black argillaceous shales passing 

 by transitional beds into the formations above and below. Jt> base lies 

 within a zone of 1 "> or '2*> feet, in which the unstratified black and white 



indurated shales that form the upper limits of the Dakota are succeeded bv 



the black clays constituting the greai mass of the Benton. The summit 

 is in the vicinity of the persistent band of light-colored limestone which 

 occurs near the base of the Niobrara and constitutes a secondary ridge 

 east of the Dakota. The area underlain by the Benton is a narrow strip 

 along the eastern base of the Dakota hogbacks, in width varying conjointly 

 with the thickness of the formation and the angle of dip, hut nowhere 

 over 1,000 feet. The thickness of the Benton at Platte Canyon, its point 

 of greatest development, is a little over 600 feet; at Deer ('reek, about 

 590; Turkey Creek, 500 ; Morrison, 580; one mile north of Morrison, L00; 

 at Ralston ('reek, 430; at Bear Canyon, '.i.\ miles south of the town of 

 Boulder, 348; and at Fourmile Canyon, at the northern edge of the held, 

 about 500. At Golden, under abnormal structural conditions, it disappears 

 entirely. 



'The lithological characteristics of the Denton formation are: a shaly 

 nature: a dark leaden, almost black, hue; concretionary clay-ironstones; 

 ami thin layers of fossiliferous limestone. 



shai«. — The shales are of line clay, with a small amount of disseminated 



arenaceous matter. They are cither jointed, concretionary, or broadly 

 homogeneous in structure. They are compact and hard, but under atmos- 

 pheric influence readily crumble to angular particles or thin scale.-. Iron 

 pyrites OCCUr disseminated throughout them in minute crystals or concen- 

 trated in thin and often finely reticulated seams. Grypsum and native 

 sulphur are also found in limited amount, the latter usually crystallized in 

 small cavities. 



ironstones. — The ironstones occur in a zone 4< I to 50 Icet thick, about 

 two-fifths the distance from base to summil of the formation. They are in 



MON XXVII 5 



