46 GEOLOGY OF TUE DENVER BASIN. 



abandoned and was replaced by the theory of tangential compression, 

 under which the more <>r less yielding interior of the mountain was, so to 

 speak, squeezed up, thus producing in effect something analogous to a 

 vertical upthrust, bill as the result of a horizontally rather than a vertically 

 acting force. Recourse has again been had by a few writers 1 to a vertically 

 acting force for the explanation of mountain uplift, but they have found 

 few followers among the actual working geologists. 



In the present case the objection to the theory that the upward curve 

 of the sedimentary strata adjoining the mountain flanks was produced 

 by the rising ,,t' the mountains, dragging up the ends of the strata with 

 them, is readily apparent mi an examination of the existing conditions 

 shown by the accompanying geological map and sections. In the first 

 place, the movement of displacement on the various longitudinal or strike 

 faults along the foothills is just the reverse of what it would he had the 

 movement been thus produced. The bedo on the side of the fault planes 

 nearest the mountains would then have been dragged up, relatively to 

 those on the other side, whereas in point of fact it is the beds on the 

 opposite side of the fault plane, or farthest away from the mountain mass, 

 that have suffered upthrow. 



Again, were the upward curve of the beds produced by the upward 

 movement of the crystalline rocks upon which they rest, those nearest 

 these rocks would have been more steeply upturned than those farther 

 away; or in other words, in a series of beds thus upturned wdiose edges 

 were subsequently planed down, the resulting outcrops would show 

 decreasing angles of dip as one proceeded eastward from the Archean 

 exposure toward the plains, whereas in point of fact the reverse is the case. 



This is shown diagrammatically in the following sketch (fig. 1), in 

 which diagram a shows the varying dips that would lie found in a series of 

 sedimentary beds that had been upturned by a vertical upward movement 

 of the Archean shore-line upon which they had rested. Diagram b, on 

 the other hand, represents the actual conditions of dip in the beds of the 

 foothill region, in which the angle of dip becomes steeper as distance from 

 the shore-line increases, and which are more readily explainable as a result 

 of tangential pressure 



Sixth Ann. Kept. I". S. Geol. Survey, 1886, p. 197. 



