84 GEOLOGY OF THE DENVER BASIN. 



tin- clearness with which the lines of stratification are delineated upon the 

 su face and the distinct tendency which they display to group themselves, 

 with respect to direction, into two well-marked assemblages — the one 

 embracing the formations of the Colorado and all below, and maintaining 

 for the greater part of their extent the same parallelism to the general trend 

 of the foothills which they have held beyond the affected area; the other 

 embracing the Montana and younger formations, and though maintaining 

 a parallelism of strike within themselves, nevertheless abutting against the 

 older formations at an angle in places as high even as 20°. The latter 

 formations, in fact, approach the range proper in a broad, well-marked, and 

 regular inward-sweeping curve, the center of its arch lying a short distance 

 north of Clear ('reek. The features just noticed again occur in a minor 

 degree and in a manner not at first liable to attract attention, in the 

 relations between the Dakota and underlying beds nearer the middle 

 of the area, where the beds of the younger formation lie across the edges 

 of those of the older. 



North of the central portion of the area of unconformity and south 

 of Ralston Creek for the distance of about 2 miles the topographical and 

 geological features are somewhat complicated by the presence of intrusive 

 masses; they arc however, still sufficiently clear to permit interpretation, 

 and with the others in the south and center of the area and in the remainder 

 of the tract to the north form one complete whole. 



Ill] FORMATIONS AND THEIR RELATIONS. 



The Archean. — This is hut slightly involved in the special geological 

 history of the region. It formed an uneven floor for the deposition of the 

 Trias, and across its truncated edges the latter formation was deposited. 



The Tria». — Iu their strike and dip the beds of both members of the 

 Trias are conformable inter se. Their strike follows approximately the line 

 of the Archean foothills, and their dip is to the east and varies between 

 35 and 90 . being shallower next the foothills, and increasing as distance 

 from them is gained. 



The lower member of this formation, the Red Beds, maintains its usual 

 appearance and, except in two places, a nearly constant thickness over the 

 entire area under consideration. The two variations in thickness an- found, 



