THE REGION Aliol'T GOLDEN. 95 



the sediments of the Montana ami overlying formations were laid down. 

 The uplift of this time was of much greater vertical and area! extent than 

 any of those which preceded it, the rise of the arch on the line of section 



given reaching at least 9,500 feet, while its lateral extent was not far from 



21 miles. It is broadly symmetrical, though then- are several sub-flexures 

 of more or less pronounced curvature. The two of greatest prominence 

 occur midway either Hank. The others, of minor development, are confined 

 chiefly to the higher part of the arch, and represent a crumpling of a 

 secondary nature along this portion of the fold. This crumpling is well 

 shown upon the present surface of the region in the changes in strike of 

 the affected beds, which are in strong contrast with the nnliroken direction 



to which the strata, of younger age hold. The possibility of the presence 

 of an occasional fault in the place of an unbroken flexure as drawn in the 

 profile is to he remarked, notably, in the vicinity of Gold Run and again 

 at points north of Coon Gulch. It so happens that here and there a space 

 intervening between two outcrops of the same bed, lying in an indirect line 

 from each other, is so covered that it is quite impossible to observe the 

 position of the underlying strata; hut since in no case a sharp break in 

 the beds of the Archcan and Trias lying below the more affected ones has 

 been discovered, it is preferable to sketch the irregularities as flexures rather 

 than as faults. 



Concerning the recognized faults in the northern and southern halves 

 of the arch, described on page 90, their true character now readily appears 

 in Profile IV, where, upon the restoration of the beds to their position in 

 pre-Montana times, the fractures are, with the local exception at the south 

 end of the Dakota hogback north of Coon Gulch, all found to lie a series 

 of slip faults of the normal type, either vertical or hading to the down- 

 thrown side and away from the center of the uplift, and similarly developed 

 on either flank of the elevation. The explanation of the normal type of 

 fault under the attendant .conditions may possihlv he found in the readjust- 

 ment of the strata, brought about by subsidence during a later period. 



The profile of this ancient hill, at least on the line given in the figure, 

 is one of structure rather than erosion, the unevenness in its outline being 

 clearly traceable to the flexures underlying, the comparatively little erosion 

 that has taken place over the higher portion of the arch having been regular 



