SUGARLOAF DISTRICT, BOULDER COUNTY, COLO. 131 



supplied pebbles for the Denver formation. In the vicinity of Boulder 

 the only direct evidence is found in connection with the rhyolite and 

 dacite. The rhyolite, which is perhaps the youngest dike rock of the 

 district, overlies the Niobrara and Pierre shales in several exposures. 1 

 This relation shows it to be at least as late as Upper Cretaceous. The 

 dacite on Flagstaff Hill was intruded before the last crustal disturbance, 

 since slickensided dacite at the contact with the sandstone is often 

 present, and thus furnishes evidence of differential movement after the 

 sheet had solidified. 



« Bulletin 265, U. S. G. S., p. 39- 



