PARK RANGE.. 133 



zontal Tertiaries cover unconformably the Mesozoic beds, so that their 

 angle of dip cannot be determined. On the west of the northern end are 

 horizontal beds abutting against the flanks of the Archaean uplift, which 

 have been referred to the Laramie group of the Cretaceous. As these beds, 

 however, show no unconformity of dip, and no great difference of litho- 

 logical character with the overlying Tertiaries, there is a possibility that 

 they may be a lower, hitherto unrecognized member of these beds. South 

 of the Little Snake River, in general, only outcrops of the Cretaceous 

 rocks are exposed along the western flanks of the Park Range; in one 

 instance only have rocks as low as the Triassic been found in contact with 

 the Archaean. The structure of these sedimentaries, as well as could be 

 seen under the great accumulation of volcanic material, is that of a series 

 of secondary rolls, approximately parallel with the axis of the range, but 

 showing no signs of having for any great extent covered the Archaean 

 rocks, since their beds, never upturned at very steep angles, are as often in- 

 clined in an opposite direction to the slopes of the main Range, as with them. 



In regard to the geological features of the Park Range, too little oppor- 

 tunity was aflforded for working out with any degree of accuracy the detailed 

 structural relations of the difierent beds. Only the broad leading features 

 that characterize the range can here be pointed out, and even these may 

 require considerable modification. 



The narrow portion from the southern limit of the map northward to 

 Davis Peak appears to have a nearly uniform monoclinal structure, with 

 the prevailing dip of the beds to the westward, which on the summit, so 

 far as observed, are inclined at low angles, falling off gently to the west- 

 ward, while near the foot-hills they dip much more steeply, passing under 

 the later rocks. The entire mountain-ridge suggests the western fold of a 

 broad anticlinal axis, whose eastern side has been carried away either by 

 erosion or downthrow. On the outlying spur which forms the northern 

 rim of the Park, west of the Platte River, near Bruin Peak, the existence 

 of this eastern side of the fold is indicated; north and south strikes being 

 observed with easterly dips. 



North of Pelham. Peak, the axis of the main fold appears to undergo a 

 somewhat abrupt change, curving round to the northwest, and striking 



