106 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



VII their relative position may be learned. We do not meet with them 

 again until we have crossed Elkkorn Gap, about 15 miles eastward. At 

 this locality we have occasion to observe very clearly the upper white 

 and pink sandstones which close the group. Its thickness amounts to 

 about 250 feet, including the light shales and marls. 



The thickness of the total group is about 650 to 700 feet. Here we 

 have arrived at the locality of maximum plication of the strata. It mani- 

 fests itself in an oval quaquaversal arrangement and in the reversion 

 of the order of succession. This quaquaversal is not the result of an 

 anticlinal upheaval alone, but is due to the anticlinal fold produced by 

 a lateral compression of the beds. Erosion has removed the apex of the 

 flexure, leaving what German geologists term an " airsaddle" (Luftsattel), 

 Section XII cuts through this point in a direction about X. 45° E., and 

 will explain the grouping of the various formations implicated in the 

 movement. 



Advancing from the northeast, we pass over the strata of the Sweet- 

 water Group (/), until we find rising before us a wall of red quartzite (c). 

 This stands so nearly vertical that it is impossible to determine toward 

 which direction it inclines. Crossing this we find the series of Carbonif- 

 erous groups (d y e, f) dipping toward the northeast. Continuing farther, 

 in a straight line, we cross successively : Trias, Jura, Dakota, Jura, Trias, 

 Jura, Dakota, Colorado. From that point drift obscures all outcrops, 

 with the exception of one red spot higher up, indicating Trias. Exam- 

 ining the dips, we find the series of beds just enumerated dipping uni- 

 formly to the northeast at angles increasing from 35° to 50°. The last 

 outcrop of Dakota to the southwest stands vertical, as do the Colorado 

 shales. It could not be determined how thick the last named were, 

 owing to an accumulation of drift. A section at right angles to the one 

 we have given would cut Carboniferous, Trias, Jura, Dakota, Jura, 

 Trias, Carboniferous. As the axis of elevation is not vertical, the Da- 

 kota and Jura would not be cut at the northwestern end of this latter 

 section. As in the previous one, so here too do we find the dip arranged 

 in conformity with the slanting quaquaversal structure. 



In the section only such portions the position of which could be defi- 

 nitely established have been directly indicated, the remaining continua- 

 tion having been given, as supposed to exist, in broken hues. We find 

 that from the northwest the strata have been overturned. Descending 

 from the highest Carboniferous point toward the southwest, we pass 

 successively over the edges of one group after the other. Beaching the 

 Dakota, we find that this is doubled. Here, then, we have a synclinal 

 fold immediately succeeded by an anticlinal. While both are due to the 

 same cause, namely lateral pressure, the horizontal dimensions of the 

 latter exceed those of the former. As soon as we have passed this 

 doubled portion we again reach Jurassic and then Triassic strata. 

 Within the latter we reach the apex of the slanting fold. Beyond this 

 the order of succession is again reversed until we reach the limit to 

 which our examinations of the surface can extend. Colorado shales, 

 standing on end, are the last group that appears clearly in this section. 

 Drift obscures all the remaining portions. Bi the section, broken lines, 

 indicating the various formations, have been extended, so as to show a 

 connection with the metamorphic granite of the ridge southward. This 

 has been done because we observe a small Triassic outcrop on the north 

 slope of the ridge, and because farther east we find the beds higher up. 

 It will be seen that a line dividing the completed folds (p q) runs 

 upward at a considerable angle. To this is due the fact that the Colo- 

 rado shales are eliminated from the first synclinal fold which doubles 



