THE REGION ABOUT BOULDER. 107 



where the outcrops arc stronger, to the smith and north, it is evidently 

 the upper beds which first disappear, the basal conglomerate being promi- 

 nently exposed, and occasionally, as well, the fire-clay in the middle of the 

 formation. Upon the formation regaining its normal development, the 

 characteristic hogbacks at once reappear. 



The Benton. — The occurrence of this formation is greatly obscured except 

 in respect tn the points at which it finally disappears, which are, respect- 

 ively, for its northern and southern segments, immediately north of Sun- 

 shine Gulch, and midway between Gregory Canyon and Boulder ('reek. 

 Its outcrops are rare, and the basis upon which it lias been represented in 

 Section III, PI. XII, as laid down against and upon the sides of the 

 Dakota elevation, is found in the stratigraphical relations which it holds to 

 the Dakota, the Dakota disappearing from the summit, while the Benton 

 still overlies — a condition which thus precludes the possibility of conformity 

 between the two formations and necessitates the deposition as figured. 



The Niobrara. — This formation, with the exception of a very short interval 

 m tlie vicinity of Boulder ('reek, was laid down entire over the area under 

 consideration. At Boulder Creek, however, the width of the outcrop is 

 considerably less than normal, and it is probable that tin' lower layers 

 were never deposited, although some portions of its basal limestone are 

 still present. 



The Pierre and overlying formations, having been laid down subse- 

 quent to the periods of unconformity in this region, will lie alluded to 

 Only as occasion demands. 



Dips and strikes of the formations. In tlleil' (lips tile formations involved ill this 



area of abnormal structure vary from 45' east, at the northern and southern 

 limits, to vertical and overthrown along the central portion. The younger 

 formations undergo the greater displacement, possibly on account of the 

 relatively greater proportion of clay beds and the consequent more yielding 

 nature of the strata, due heavy sandstones of the Red Beds alone have 

 everywhere dips less than 90°. The general easterly dip of the strata is 

 regained in the Montana, from a half mile to 1 mile east of the range. 



The strikes of the several formations present relations with each other 

 which are, in a high degree, significant of the history of the region. The 



