22 GEOLOGY OF THE DENVER BASIN. 



effect is more pronounced on the western than on the eastern slopes of the 

 region. On the eastern shore of the Colorado Range the movement and 

 the succeeding erosion were so uniform that it is difficult to detect any 

 discrepancy of angle either in strike or dip between Triassic and Jurassic 

 beds. In the area mapped, however, the effect of the movement is shown 

 in the Golden arch, whose axis runs just north of Clear Creek, between 

 it and Gold Run. The crest of this arch and the Triassic beds already 

 deposited over it were raised about 420 feet by this movement, and by 

 the subsequent planing of the crest a great portion of the upper Wyoming 

 formation, down to the limestone near its base, was removed. That the 

 absence of these beds from where the arch once existed is due to erosion 

 rather than to nondeposition is proved by the fact that as one follows the 

 strike of the present upturned beds toward Golden from either direction, 

 the beds of the upper"Wyoming formation disappear successively from the 

 top downward, and the discrepancy in strike is between them and those 

 of the next succeeding higher horizon. The movement is also assumed to 

 have produced a similar though less pronounced arch in the vicinity of 

 Boulder, from the crest of which the upper Wyoming formation was eroded 

 off and over which the succeeding Morrison beds were not deposited. The 

 same general phenomena obtain in these beds in their present outcrops as 

 near the Golden arch. It was for this reason that the upper beds of this 

 formation, which from their lithological constitution were formerly consid- 

 ered Jurassic, have been classed by Mr. Eldridge as Triassic, 



MORRISON FORMATION. 



The beds of this formation, which were deposited during the general 

 depression that followed the Jurassic movement, consist mainly of marls 

 with varying proportions of sandstones and thin limestones, the whole 

 having an average thickness of 200 feet. Lenticular bodies of drab lime- 

 stone occur in the clays of the lower two-thirds of the formation, which 

 also carry the remains of gigantic saurians characteristic of the formation, 

 and from which the name "Atlantosaurus beds" has been given to it. The 

 upper third of the formation is more arenaceous, sandstones sometimes 

 predominating over the (days and passing into a conglomerate at the base. 



