716 VICTOR ZIEGLER 



geology of the Denver basin. 1 The following description is taken 

 from the latter publication: 



The normal appearance of the foothills is that of a mountain mass of 

 Archean rocks, fringed at an average distance of one half or three quarters 

 of a mile by a sharp serrated ridge of Dakota sandstone, the valley between the 

 two being occupied by the formations of the Trias and Jura. Above the 

 Dakota come .... the Benton, the Niobrara — this generally constituting 

 a second smaller reef outside the Dakota — the Pierre, the Fox Hills, and the 

 Laramie, the basal sandstones of the Laramie again forming either a low roll 

 in the ground or an actual comb of rock slightly projecting above the surface 

 of the surrounding prairie. To the east of the Laramie .... appears in the 

 southern portion of the area yet another comb formed by the conglomerates 

 at the base of the Arapahoe series. Finally this is followed by ... . the 

 Denver formations. 



To this description it is well to add that as a general rule the 

 Lyons formation forms a prominent, though low, hogback in the 

 strike valley to the west of the Dakota, and that in the northern 

 part of the foothills the Archean-Fountain contact forms a promi- 

 nent strike valley, and that here the Fountain is characteristically 

 developed into a high, precipitous hogback, usually capped on its 

 crest and dip slopes by the Lyons sandstone. Here also the 

 Arapahoe and Denver formations are absent, and the Laramie 

 lies almost horizontal twenty miles or more to the east of the 

 foothills. In the southern part of the area the dips in the Foun- 

 tain, Lyons, and Lykens average from 35 to 50 . These increase 

 gradually eastward until they become vertical or even overturned 

 in the Laramie, dips as low as 75 west being noted. Farther 

 eastward these flatten within a few hundred feet from vertical 

 into practically horizontal in the upper part of the Arapahoe or 

 the base of the Denver. 



No such variation is noted in the northern part of the area 

 under discussion. West of Loveland dips of 40 are rare and 

 occur only locally as the result of special conditions. The steepest 

 dips occur here in the Dakota or Morrison. It is also worth noting 

 that in the southern part of the area, where steep and overturned 

 dips are met with, the formations have turned practically hori- 

 zontal within two miles of the Archean contact — in one extreme 



1 U.S. Geol. Survey, Monographs, XXVII, 1896. 



