TOPOGPAPHY AXD GEOLOGY NEAP BOULDER. 105 



THE REGION ABOUT BOULDER. 



The structural features of the region about Boulder arc of the same 

 general type, except for the absence of eruptive phenomena, as at < irolden — 

 that is, there is here another scries of unconformities occurring at various 

 horizons from Archean to Montana. The area affected by this scries of 

 unconformities extends along the foothills from a point a little over "_' 

 miles south of North Boulder ('reek to one about 1 .', miles north: its 

 breadth is nowhere much over 1 mile, and the phenomena arc confined to 

 the open slopes of the foothills and the bench lands and prairies adjoining-. 



TOPOGRAPHICAL FEATURES. 



As about Golden, the topographical features, through influence of the 

 geological phenomena involved in the region's development, again diverge 

 from normal. Instead of the fringing reefs of Dakota and Niobrara, the 

 Triassic and Archean slopes directly overlook the prairie. The disappear- 

 ance of the hogbacks, the wavy trend of the strata in actual outcrop or in 

 soil delineations, the approach of formations in converging strikes, and the 

 occurrence of short east-and-west faults all contribute to variation from 

 the usual foothill character. 



GEOLOGICAL FEATURES 



The formations involved in the geology of the area include all that 

 occur in the Denver field from Archean to Picric. 



The Archean. — This presents the usual waw line of union with the 

 Trias, significant of the unevenness of the floor receiving the deposits of 

 the latter, but no feature of structure or sedimentation peculiar to this 

 region alone appear.-. 



The Trias. — The lower member, so far as the phenomena under discussion 

 are concerned, has been unaffected, unless, perhaps, in the thinning which 

 is observed in the vicinity of Boulder Creek, due to a rise in the Archean 

 floor, which may have been the incipient movement of the series to follow. 

 The local fold developed on the face of the Triassic slopes between Boulder 

 ('reek ami Gregory Canyon is of an origin later than the phenomena here 

 discussed, and unrelated to them. 



