DEARTH OF EVIDENCE OF UNCONFORMITY. 253 



are characteristically represented by the great plain areas of the present da v. 

 In the former, the strata show the effects of powerful and repeated tangential 

 compression, not only in their steeply inclined positions and sharp folds and 

 faults, but in the frequent and marked angular unconformities between beds 

 deposited before and after an orographic movement. In the latter, on the 

 other hand, the inclinations of the strata diverge but little from a horizontal 

 position, the folds are but gentle undulations or monoclines broken by 

 faults of moderate displacement, and no angular discordances between suc- 

 cessive strata are to be observed, whatever orographic disturbance may have 

 intervened between the times of the respective depositions. 



Nowhere is this change of condition more marked and sudden than in the 

 Rocky Mountains of Colorado. In leaving a mountain area one may pass 

 in a mile or two from steeply upturned and even reversed strata, showing 

 evidences of violent movements accompanied by long periods of erosion 

 before succeeding beds were deposited, to an adjoining plain where the same 

 beds rest in horizontal position and in perfect stratigraphical accordance one 

 over the other, and where the only evidence of erosion on the beds below 

 the horizon of the movement may be a variation in their asrarregate thick- 

 ness. Not only is this true of the outer flanks of the mountain ranges, but 

 it can also be observed to hold good for many of the interior depressions 

 which would seem to have been either valleys or arms of the sea throughout 

 the various phases of the geological evolution of the region. 



It is evident, therefore, that except in highly disturbed regions actual evi- 

 dence of unconformity must be extremely rare, the parallel succession of 

 beds after an orographic movement, or parallel transgressiou as it is desig- 

 nated by European geologists, being far more common than actual discord- 

 ance of stratification ; but even in highly disturbed districts, I have found 

 that a very marked discordance of stratification is not always shown by an 

 actual angular unconformity along the line of dip, but that its evidence is 

 readily found only in variations in the strike between beds deposited before 

 and after an orographic movement, or, what amounts to the same thing, by 

 the observation that the later beds rest at different points upon different 

 horizons of the earlier series of beds. The explanation of an extreme case 

 of conformity in angle of dip, combined with the greatest variations in strike, 

 which has come under my observation, is very readily apparent and, with 

 local modifications, is doubtless applicable to all similar structural phenom- 

 ena. In the given case, the beds already deposited were by an orographic 

 movement thrown into a series of folds whose axes had a general east and 

 west direction. After the crests of these folds had been planed off by erosion , 

 a second series of beds was deposited upon them, producing a complete suc- 

 cession of beds with no discrepancy of angle, along an east and west line in 

 the troughs of the synclinal folds, but with gaps of varying width in the succes- 



