198 DESCRIPTIVE GEOLOGY. 



SECTION II. 



GENEEAL GEOLOGY. 



The Uinta Uplift. — The geological structure of the Uinta Range, 

 like its physical structure, is in its main outlines remarkably simple and 

 regular; its main mass was formed by the uplifting of an immense thick- 

 ness of practically conformable strata, at the close of the Cretaceous period, 

 in a broad anticlinal fold, having the form of a flat arch, or inverted 

 U. The movement which produced this fold was accompanied by com- 

 paratively little fracturing and dislocation of the strata, and by no intrusion 

 of eruptive rocks. By subsequent erosion, during Tertiary and recent 

 times, there has been removed, from the crest of this fold, a thickness of not 

 less than 20,000 feet of rock strata, whose upturned edges, in a great 

 measure buried beneath the Tertiary beds which have been formed from 

 their d(ibris, encircle in parallel curves the present flanks of the range. 



In the eastern part of the range, this simple structure was somewhat 

 complicated by the presence of a preexisting ridge of Archaean rocks, which, 

 though probably covered by the conformable series which had been 

 deposited unconformably aroimd and over it, was more or less involved in 

 the main Uinta uplift. Of these Archaean rocks, only a comparatively 

 small area is now exposed, and, of their structure, little can be learned 

 except that it is extremely complicated. They consist, in general, of 

 quartzites, Avliite mica and hornblende-schists, with a local development of 

 paragonite beds, and correspond most nearly to those classed as Huronian 

 in the Rocky Mountains. 



The conformable series of beds involved in the Uinta fold extend in 

 geological horizon, as far as their age has been satisfactorily determined, 

 from the Carboniferous up to the top of the Cretaceous. The Carbon- 

 iferous formation, whose beds form the main mass of the range, is represented 

 by the groups of the Weber Quartzite, Upper Coal-Measures, and Permo- 

 Carboniferous. The beds of the first of these form the crest throughout 

 the greater part of its extent, lying nearly horizontal, or inclined at very 

 low angles. They consist of a lower series of white and reddish, compact 



