STRUCTURAL QEOLOGY. 43 



In the Denver Basin the beds underlying the plains area, except for 

 some slight fracturing in the developed coal beds of the northwestern por- 

 tion of the area, are practically in the position in which they were origin- 

 ally deposited They have been lifted above sea-level and exposed to 



erosion at various times, and there is possibly a slight upward curve of the 

 Mesozoic beds on the eastern edge of the basin sufficient to produce a 

 basin structure, but of deformation or pronounced flexing of the beds there 

 is, so far as observed, a notable absence. 



In the foothill region, on the other hand, there is pronounced defor- 

 mation, manifested in both folding and faulting, but it has a character of its 

 own, distinct from the intense plication found in highly disturbed mountain 

 regions, though partaking- of some of its characteristics. It occupies struc- 

 turally, as it does geographically, an intermediate position between the 

 relatively undisturbed areas of the plains and the intensely disturbed and 

 plicated rocks of the mountain region. 



The Archean areas of the present region, as has already been stated, 

 have not been specially studied; hence nothing need be said of their struc- 

 ture except that they were already so intensely deformed prior to the 

 deposition of the Mesozoic beds that it is highly improbable that any study 

 of their present structure would enable one to trace the effects of the later 

 movements which have affected the more recent beds, though these move- 

 ments undoubtedly must have been felt within the crystalline complex. 



PLAINS AREA. 



The departure from a horizontal position of the strata on the plains is 

 usually so slight as not to be susceptible of instrumental measurement, so 

 that it must be measured by the relative position of outcrops of the succes- 

 sive beds as platted on an accurate profile. Such profiles, given on the 

 structure sheet (PI. IV, in pocket), which are limited in extent to the width 

 of the map, also show no very perceptible rise of the strata to the eastward, 

 and from these one would judge that the basin in which the Arapahoe and 

 Denver beds was deposited was a basin of erosion. In crossing the plains 

 still farther east, though the surface rises slightly, one crosses successively 

 lower outcrops of the Laramie strata as one goes eastward: hence there 

 is apparently a slight rise or arching up of the strata toward the present 



