42 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



by this separate elevation of the Palaezoic rocks above tbe adjacent 

 Cretaceous strata which now cover the less uplifted part of the Axial 

 Flexure reached from 7,000 to 8,000 feet, and the displacement was 

 accomplished partly by faulting this immense thickness of strata and 

 partly by very abrupt flexure. 



The eastern end of the mountain portion of the Axial Uplift termi- 

 nates by a broad, sweeping, or partiversal* dip of the strata, which dip 

 is abrupt, much like that of the sides of the uplift. That part of the 

 terminal mountain mass of the main range which lies within and upon 

 the northern boundary of this district consists of Carboniferous strata, 

 and is flanked at its base by the upturned edges of the Jura-Trias, Da- 

 kota, and Colorado groups successively. The strata of the Colorado 

 Group, however, quickly become horizontal, or nearly so, and occupy the 

 low ground around the mountain ; but they are there partly obscured by 

 the strata of the Uinta Group. A very large part of the mass of the 

 Uinta iMountain Range consists of the Weber quartzite, which great 

 formation is probably about fifteen thousand feet thick. AH the visible 

 portion of the Axial Uplift, however, that lies within this district, con- 

 sists of Carboniferous strata alone, except the Weber quartzite, which 

 is exposed in Junction and Yampa Mountains, presently to be described ; 

 and is also, perhaps, very slightly exposed at the point where Signal 

 Shot Creek Caiion opens into the eastern portion of Red Rock Basin. 



Junction Mountain JJpthrust. — As regards their structure and origin, 

 Junction and Yampa Mountains are most remarkable isolated mountain 

 masses, both having essentially the same structure, and doubtless a 

 simultaneous origin. Both are situated upon the axis of the Axial 

 Flexure, eastward of and separate from the main mountain masses of 

 the Uinta chain and from each other, and where that flexure is compara- 

 tivelj" slight. 



Junction Mountain is situated three or four miles east of the terminal 

 mountain mass of the Uinta chain, between which two mountains Snake 

 River flows to its confluence with the Yampa. Although this mountain 

 is so near the other mountains of the Uinta system, and is so evidently 

 a part of that system, it is, nevertheless, entirely isolated in its struc- 

 ture and elevation as well as by its position. It consists of a separate 

 and distinct upthrust of the Carboniferous and Weber quartzite strata 

 through those of Mesozoic and Cenozoic age, which have remained 

 almost undisturbed in the immediate neighborhood by that separate 

 and remarkable displacement. The manner of this displacement, which 

 I have called an upthrust, may be illustrated by the action of a large 

 13uuch, worked by machinery, for perforating heavy iron plates, so 

 clearly defined does the separation appear to be between the uplifted 

 and the surrounding strata. The illustration will doubtless be more ac- 

 curate if we imagine the punch and die to have become so worn by use 

 that the iron plate is torn in places and nowhere clearly cut in the 

 process of punching. This mountain upthrust is oval in form, the long 

 diameter being nearly twelve miles and the short one about four miles. 

 The strata about midway of both sides are nearly or quite vertical, 

 while the dip at the northern and southern ends, although steep, is much 

 less than at the sides. 



The direction of the long diameter is northwestward and southeast- 



*A11 anticlinal axes must necessarily dip more or less in some portion of their ex- 

 tent; but in this region there arQ many examples of very short and rapidly-dipping 

 anticlinals around the vanishing end of which the strata dip by a sweej) of the greater 

 or less part of a circle. I apply the term " partiversal " to such a dip, and use it in a 

 sense similar to that in which " quaquaversal " is used for a dip in all directions. 



