346 UNIVERSITY OF COLORADO STUDIES 



is deemed an indication of tangential compression. Two diagrams 

 are given to show the different effects of vertical upward and oblique 

 downward pressure, which are here reproduced, Fig, 1. 



Acceptance of that idea without further investigation led the 

 writer and others at first to overlook certain phenomena, until the 

 discovery of what appear to be Benton shales on the south side of 

 Boulder Creek, disappearing under the apparently overturned Jura- 

 Trias at the base of Flagstaff Mountain (the axis of the Boulder 

 Arch, described in the monograph before mentioned), compelled a 

 re-examination of the subject. In this vicinity the most pronounced 

 overturn is in the Niobrara basal limestone, which is very hard and 

 sufficiently resistant to form a ridge-making element. It is normally 

 overlaid by several thousand feet of easily eroded Upper Niobrara 

 and Pierre shales, and underlaid by Benton shales. When these 

 formations are erected to a position approaching the vertical, the 

 rapid cutting away of Upper Niobrara and Pierre shales must inevit- 

 ably leave the Niobrara limestone partly unsupported on the east 

 side, to bear the burden of the lateral pressure of the mountain 

 column upon its base. Flagstaff Mountain rises abruptly about 

 1,000 feet above the upturned edge of the limestone. Other foothills 

 are still higher, others still are lower and less abrupt, while beyond 

 the foothills the main Rocky Mountain range towers to a height of 

 from 10,000 to over 14,000 feet above the level of the sea. 



There are reasons for supposing that at Flagstaff, as the unsup- 

 ported limestone gave way and overturned, a break in the underlying 

 Dakota (here very thin) and in Jura-Trias permitted the latter to 

 swing outward at the base and inward and downward at the apex, 

 thus executing a partial revolution on an axis. In the meantime, the 

 yielding Benton shales crowded down into the opening thus made, 

 and the broken edges of the Triassic, swinging outward, passed out 

 over the Dakota, Benton, and Lower Niobrara in such a position as 

 now to rest upon the overturned Niobrara shales, giving the impres- 

 sion at first glance that the Dakota, Benton, and Lower Niobrara had 

 never been deposited, and that the Triassic had participated with the 

 Upper Niobrara in the overturn. The following diagram, drawn by 



