62 chology of the denver basin. 



The bed is essentially a sandstone, but is divided into minor layers by 

 bands of hard, white clay from li to 2 feet thick. Occasionally the clay 

 also is specked with rust spots, and upon becoming coarser-grained is 

 directly identifiable with the mottled or specked sandstone described above. 

 It is also sometimes conglomeritic. 



The cause of the variation in the thickness of the upper half of the 

 Jurassic could not be determined from conditions existing in the Denver 

 field, but an oft-suggested unconformity at the base of the Dakota may be 

 the explanation. 



CRETACEOUS. 



Wherever along the foothills of the range the Mesozoic beds have 

 been upturned at a comparatively high angle of inclination, the different 

 Cretaceous formations may be recognized by their topographic features 

 and surface relations. The Dakota sandstone, dipping east, forms a promi- 

 nent line of "hogl lacks "—sharp, serrated, monoclinal ridges, that extend 

 with occasional interruptions along almost the whole front of the range. 

 The black, slaty shales of the Benton occupy in general a shallow longi- 

 tudinal depression, from 400 to 1,000 feet wide, between the Dakota terrane, 

 and the white basal limestone of the Niobrara, which forms a second ridge, 

 much lower than the "hogbacks," but still conspicuous. East of the Nio- 

 brara are the clays of the Pierre and Fox Hills, underlying a broad flat 

 belt 1 to 2 miles wide, succeeded by the basal sandstones of the Laramie, 

 which outcrop in low, somewhat irregular combs. Between the basal 

 sandstones of the Laramie and those of the Arapahoe, next overlying, is 

 another flat, from 600 to 1,200 feet wide, occupied by the (days of the older 

 formation. The basal sandstones and conglomerates of the Arapahoe form 

 local crests 10 to 20 feet above the adjoining prairie, east of which the 

 strata gradually assume a gentle easterly dip and the surface features are 

 chiefly those of erosion in approximately horizontal strata. 



DAKOTA FORMATION. 

 STRATIGRAPHY. 



This formation is from 22f> to 350 feet thick, and usually consists 

 of two or three nearly equal benches of massive sandstone separated by 

 narrow hands of shale which locally become fire-clays. A characteristic 

 conglomerate occurs at the base of the formation; at the summit, a zone 



