46 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



and, passing through the southern end of Pinon E-idge, ir crosses White 

 Eiver and quickly disappears by a partiversal dip of the strata of the 

 Laramie and Wasatch groups. The extent of the displacement of 

 this eastern portion of Midland Uplift is much less than that of the 

 western portion, and has brought to view no strata beneath those of the 

 Cretaceous groups. 



Midland Uplift consists essentially of a monocliual tlexure, with the 

 rise upon the northern side. The flexure is quite as distinctly mouocli- 

 ual in the less displaced eastern portion as it is in the western part, where 

 the northern side of this uplift is in contact with the southern side of 

 Plateau Uplift. The facts observed seem to warrant the conclusion that 

 all the flexures of the strata which pertain to the Uinta system within 

 and adjacent to this district were simultaneous in their origin, and that 

 the movement of elevation of these folds proceeded simultaneously up 

 to a certain stage and then halted in their common upward movement. 

 Also that the special elevation of the mountain masses, which occupy 

 either the whole or the principal part of each of the original folds, then 

 took place as sharply-defined superadditions to their iuilial elevation, 

 which culminated in the Uinta Mountain system. It is also evident 

 (assuming for purposes of description the actual uplift of these folds) 

 that certain of the specially -elevated accessory uplifts halted in their 

 upward movement, while that of the adjacent larger or principal ones 

 continued. This view is supported by the fact that all along the flex- 

 ure which divides Midland Uplift from that of Yampa Plateau the 

 strata of the former are sharply flexed up against the latter, presenting 

 the appearance of having been dragged by the superadded upward 

 movement of the Pateau uplift. 



Midland Uplift terminates at its western end by narrowing rapidly to' 

 a point against the southern side of Section Ridge, so that the Creta- 

 ceous strata, which are flexed uj) against its own south side, are con- 

 tinued in like manner without interruption or material change in dip 

 along the south side of Section Ridge and around its western end, as 

 will be further explained under the head of flexures. 



Raven Parle Uplift. — The middle of this uplift is some eight or ten miles 

 south of the southern border of Midland Uplift. It is thus not only iso- 

 lated from all those that have been before considered, but its elevation 

 has involved a m.uch less amount of displacement of the strata of the 

 region than that of any of the others, the lowest strata that have been 

 brought to the surface upon its axis being those of the Colorado Group. 

 In this respect it resembles the less elevated eastern portion of both the 

 Axial and Midland Uplifts, but it is free from any superadded uplift or 

 upthrust, such as has taken place upon those primary uplifts. By this 

 characteristic, as well as by its local separation and slight relative ele- 

 vation, it is distinct from the Uinta Mountain masses, but it is never- 

 theless an outlying fold belonging to that system of uplifts. 



Raven Park Uplift is a very short one, not more than twenty-five or 

 thirty miles long, its eastern and western extent being scarcely so great as 

 that of Midland Uplift, from which Raven Park Uplift lies immediately 

 southward. Its longer axis, however, is not parallel with that of those 

 portions of the three uplifts that lie directly north of this uplift, but its 

 direction is northwestward and southeastward; in this respectconforming 

 iu general direction with the southerly sweep of the eastern end of both 

 Axial and Midland Uplifts. The significance of this change in the direc- 

 tion of these axes from that of the general trend of the Uinta Chain is 

 doubtless to be sought iu the fact of their approaching relation to the 

 uplifts and flexuresof theParkRangesystem. The flexure of the southern 



