44 REPORT UNITED STATES GEOLOGICAL SURVEY. 



about sixteen miles eastward from that of Junction Mountain, with its 

 long diameter upon and directly across the low, eastward extension of 

 the Arial Flexure of the Uinta Range. Its long diameter has a direction 

 a few degrees west of southVard, and, therefore, it does not correspond 

 with the axes of any of the other uplifts of the Uinta system. Upon 

 casual view it might be expected that the long diameters of both these 

 upthrusts would be found to correspond with the axis of the Uinta 

 Eange, or at least that they would corresjiond with each other, since 

 the two mountains are so similar, and both occur on the same axial 

 flexure. On the contrary,if lines drawn through the long diameter of each 

 of the upthrusts were produced southward, they would meet at an angle of 

 about 50O. The relative position of the long diameters of these upthrusts 

 is in keeping with the curvature of the Axial Flexure upon which they are 

 located, as it sweeps around from the Uinta to join the Park Range 

 system ; and their transverse instead of longitudinal position upon that 

 flexure is probably due to simultaneous impingement of force that was 

 exerted from both the adjacent mountain systems while the upthrusts 

 were in progress as superadditions upon the original flexure. 



The strata of the Colorado Group which surround Junction Mountain 

 are much obscured by the overlying drift and the soft strata of the Uinta 

 Group. The strata of the Colorado Group also closely surround Yampa 

 Mountain, and are also largely obscured by the same deposits as in the 

 other case. 



The escarpments of the Fox Hills and Laramie groups that border 

 both sides of the long Axial Basin reach within the immediate vicinity 

 of Yampa Mountain ; but the strata of the escarpment upon the south- 

 ern side, while they partake of the general axial flexure, do not seem te 

 have been especially flexed or otherwise disturbed by the extraordinary 

 movement of this upthrust except at its immediate borders. Those on 

 the northern side, however, are found to have been abruptly flexed to 

 the northward by the less abrupt elevation of the strata composing the 

 northern portion of the upthrust. The character of these two upthrusts 

 is partially illustrated in the long section at the bottom of the geological 

 map accompanying this report, and still further by the longitudinal 

 section of Yampa Mountain upon the same sheet. 



The Plateau tlpHft. — The long, broad mountain mass which has been 

 designated as Yampa Plateau upon the accompanying and other maps, 

 is not strictly parallel with the axis of the adjacent portion of the main 

 uplift of the Uinta Range, yet it is, in an important sense, a parallel 

 and accessory flexure. It is not, properly speaking, divergent from the 

 main axis of the range, but by its northern side it lies closely adjacent 

 to, really parallel with, a corresponding portion of the southern side of 

 the main uplift, with which it in part coalesces; and yet the Plateau 

 Uplift is quite independent in its termination at both ends, as well as 

 nearly or quite so by a separate svnclinal flexure. In a mere topograph- 

 ical sense. Midland Uplift also constitutes a part of Yampa Plateau, but 

 for geological purposes that uplift must be separately considered. In 

 some degree Plateau Uplift is an epitome of the great Axial Uplift, for 

 it also consists of the foil series of Carboniferous strata together with 

 a central mass of the Weber quartzite. Its manner of uplift is also 

 similar, for it rises by an abrupt flexure of the strata upon either side, 

 the flexure of the strata between being comparatively slight. This 

 character, as well as the relations of the Plateau Uplift to its associated 

 displacements, is shown in one of the sections at the bottom of the ac- 

 companying map. The flattening of the flexure between its two ab- 

 ruptly-bent sides gives the uplift its plateau-like character, which, how- 



