THE OVERTURNS IN THE DENVER BASINS. 



Geological field work in the foothill region of the Denver 

 Basin has possibly been hampered to some extent by the assump- 

 tion that the overturn of certain formations — and the locally 

 increased dip of the higher strata in other cases — were caused by 

 the tangential, or nearly horizontal, pressure which is commonly 

 supposed to have produced the mountain range. It is very possible 

 that the direct effect of gravitation has not received sufficient 

 consideration. Without in the slightest degree discrediting the 

 lateral-compression theory of mountain uplift, of which there is 





Fig. I (after figure in Monograph XXVII, U. S. Geol. Survey, p. 47).— ^ shows 

 effect of vertical upward pressure, with dip of strata nearest the point of uplift greater 

 than at a more distant point. B shows effect of tangential pressure. 



Other evidence along the foothills, it is worthy of notice that the 

 overturning of strata flanking the foothills, may, at least in many 

 places, and in every instance with which the writer is familiar, 

 be, with good reason, ascribed to a very different cause. 



In Monograph XXVII, United States Geological Survey, 

 Geology of the Denver Basin, the fact is pointed out that the 

 strata at some distance from the Archaean are generally tilted 

 to a much higher angle than those lying nearer the granitic axis 

 of the range, and this is deemed an indication of tangential com- 

 pression. Two diagrams are given to show the different effects 

 of vertical upward and oblique downward pressure, which are 

 here reproduced. Fig. i. 



Acceptance of that idea without further investigation led the 

 writer and others at first to overlook certain phenomena, until 



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