156 ROLLIN T. CHAM BERLIN 



pre-Cambrian complex, while the porphyries were as obviously 

 derived from the flows of the Middle Park formation itself. The 

 old granites and the Tertiary effusives are in nearly equal pro- 

 portions. Above this conglomerate for another thousand feet are 

 various arkosic sandstones and coarse conglomerates whose material 

 has been derived from both the old granites and the young andesites. 

 Pebbles of white and purplish vitreous quartzite are conspicuous 

 in places. 



The import of this formation is that, either accompanying a 

 late stage of the post-Laramie folding or soon after it, there were 

 extensive outbursts of andesite in this portion of the Colorado 

 Rockies. These flows were either so vast or else were so located 

 relative to the basins where sediments accumulated, that they 

 furnished a very large part of the detritus which made up the lower 

 portion of the Middle Park formation, as urged by Cross, while 

 the Archean granite contributed practically nothing. But later, 

 after these volcanics had suffered much erosion, or else there had 

 occurred further uplifting along the granite core of the range, or 

 both, the granite areas supplied much coarse pebble and cobble 

 material to the sediments which were accumulating in this basin. 



Early Tertiary folding. — Since deposition this Middle Park 

 formation has suffered a period of folding which has accentuated 

 the structural basin by causing the basal volcanics on the eastern 

 margin of the syncline to dip westward at angles of 5o°-6o° and the 

 same series on the western margin to dip eastward at angles up to 

 30°. The upper beds of the formation exposed in the middle of the 

 syncline possess only very gentle undulations which cause them to 

 depart but little from a horizontal position. 



The time of the folding movement which disturbed these beds 

 was not closely determined in this present study. In any case it 

 was post-Middle Park and pre-Uinta, for after the folding a long 

 period of denudation had greatly changed the region before the beds 

 classed as Uinta were laid down. It is possible that this is to be 

 correlated with a prominent folding period which has been recog- 

 nized in the mountains of Wyoming, and which can be more closely 

 timed. There the folding and faulting occurred betAveen the 

 Fowkes formation of the Lower Wasatch and the Knight formation 



