•_'7<l S. I'. EMMON OROGRAPHIC MOVEMENTS. 



( )n tin' eastern shores of the < lolorado island no evidence of the existence 

 of marine Jura has been found south of the Latitude of the Laramie plains. 

 The fresh-water beds rest directly upon the Triassic without any apparent 

 discrepancy of angle. The thickness of" Red Beds" assigned to the latter 

 age varies very greatly from poinl to point. This would naturally be ex- 

 plained by the unequal erosion of these beds during their elevation ; but 

 where the evidence of sub-aerial erosion seems insufficient it might be partly 

 accounted for in the case of beds, which like these bear internal evidence of 

 having been deposited in strong along-shore currents, by the existence of 

 broad, ridge-like corrugations in the sea bottom extending out at an angle to 

 the shore-line, on the crests of which the accumulation of sediment would be 

 much less than in the adjoining depressions. There is sonic evidence of the 

 formation of such corrugations during the movement of elevation at various 

 points along the eastern front of the mountains, though it cannot always be 

 definitely assigned to this period. 



In the Canon city region there is evidence of considerable elevation and 

 erosion during the movement, followed by a subsidence which admitted the 

 Jura-Dakota waters to Webster park and to the valley of Parkdale at the 

 west end of the Royal gorge. How far these waters extended to the 

 northwest towards the South park depression has not yet been determined. 

 Near Canon City the discordance of strike between the now sharply up- 

 turned Jura-Dakota and the underlying beds is most marked, and points to 

 a very considerable disturbance and erosion of the latter before the former 

 were deposited. As the immediately underlying beds are here very Bofl and 

 easily eroded, the actual contact and any discrepancy of dip-angle that may 

 exist with these intermediate beds, whether Carboniferous or Triassic in B 

 has not been observed, due dura Dakota beds rest at different points, 

 however, on these, on the early Palaeozoic beds, or on the Archaean; and 

 their discrepancy of angle with the two latter is very marked. 



The western shore-line of the Colorado island is more difficult to define 

 than the eastern, since it has been more extensively faulted and eroded in 

 post-Me80zoic tim< - 



It is noticeable that the northwest structural line along which the greatest 



disturbance ha- taken place passes thomgh the ("anon City region just de- 



ribed. The most notable effect of the orographic movement along this line 

 was the cutting off of the previously existing connection between the South 



park bay ami the western ocean of the Plateau region, an effect which ha- a 



more than local significance. It was produced by an uplift of the northern 

 P irtion of the Mosquito range and of the (; lie mountains on the east side 

 of the Mosquito fault, which has been traced northward along the western 

 crest of the Mosquito range and t hence northwestward along the wesl tlanks 

 <»f the Gore mountains to within fifteen or twenty mile- of the Grand 



