'_!•"! S. I'. EMMONS — OROGRAPHIC MOVEMENTS. 



Bion of beds on the crests of the anticlinals. In the following movemenl 

 both series were thrown into a series of folds the prevailing direction of 

 whose axes was aorth and south, or at right angles to the preceding folds ; 

 and after these folds had been eroded, in the beds left standing with a steep 

 western dip, the evidence of the earlier folds is found only in their irregu- 

 larly-waving line of strike as compared to the i iparatively Btraight one of 



the later beds, while the angle of dip in the two series is in many cases per- 

 fectly conformable, and what variations may exist in other cases is generally 

 undistinguishable, either from its Blight amount or from the unfavorable 

 position of the exposun 



In weighing the evidence for or against an orographic movement in a 

 given region it would seem, therefore, that the positive proof afforded by one 

 or two instances of unconformity should overbalance the negative testimony 

 of many instances of apparent conformity. 



In endeavoring to trace out the orographical history of the Rocky .Mount- 

 ain region I have followed the method of reconstructing in my mind the 

 probable outlines of its various land-masses when a period of sedimentation 

 began after the close of an orographic movement, and the changes produced 

 in those outlines by each succeeding movement. 



Rocky Mountain Region. — The mountain area which is referred to in this 

 paper as the Rocky Mountain region, is a north and south belt about 150 



miles in width, extending from northern New Mexico through the State of 

 Colorado into southern Wyoming, a distance in round numbers of about 

 100 miles. As the land areas at the close of the successive movements espe- 

 cially referred to correspond more or less closely to the areas of the principal 

 mountain ranges, areas whose general lines of uplift it may be assumed were 

 determined very early in its history, perhaps at the close of the Archaean, 



they will he referred to as islands under the name- that are now -applied 



to the ranges. Their general disposition is as follows: The mountain uplift 

 fronting the Greal Plains, which as a whole has a meridional trend, is divided 

 by depressions having a general northwest trend into three more or less dis 



tinct ranges, whose northern continuation.-, leaving the line of uplift which 



fronts the Plains, trend to the northwesl and thus produce :i structure en 

 echelon for the whole system. The northern and most extensive of thes 

 the Colorado range, extends from Pike's peak northward to the Colorado 

 Btate line and then splits int.! two distinct uplifts on either Bide of the broad 



elevated valley known a- the Laramie plains. The eastern of the86 uplifts, 



tie' Laramie hill-, was a submerged reef in Palaeozoic times and has a Bome- 

 what broken connection by -mall projections of Archaean exposures with the 

 Black Hills of Dakota. The western uplift, known a- the Medicine Bow 

 range, trends northwestward between the Laramie plains and the North 



park, at one time having been connected with the northern end of the Park 



