EARLY PALEOZOIC SEDIMENTS. 17 



the Rocky Mountain region is due to a depression of the ocean-level or an 



elevation <>t' this region after Silurian time. 



Within the area mapped no outcrops of Lower Paleozoic beds are 

 found, bui there is good reason to assume that they underlie all the later 

 sediments and do not reach the surface simply because they are concealed 

 along the Archean contact by the overlapping of Mesozoic and later 

 deposits. A short distance south of this area, however, in Perry or Pleas- 

 ant Park, at the base of the Archean foothills, is a small exposure of 

 Paleozoic beds whose exact horizon has not been determined, hut' which, 

 from descriptions and analogy with other exposures, are assumed to belong 

 to the Lower Paleozoic series. The overlap of the Mesozoic Over the edges 

 of the older beds is there visihlv demonstrated. 



CARBONIFEROUS MOVEMENT. 



At some time during the Carboniferous period not vet definitely 

 determined, hut probably during the latter half, an important orographic 

 movement took place in the Rocky Mountain region which in certain 

 localities was accompanied by some dynamic disturbance. Its most ^•ii- 

 eral effect was an elevation of the land, raising above water-level portions 

 of the regions that were previously submerged and exposing them to 

 subaerial erosion. In some cases it would appear that entirely new land 

 masses were formed which in later movements were again submerged, and 

 that in other cases land masses were depressed so as to lie subject to 

 sedimentation which had received no sediments during the early Paleozoic 

 cycle. The movement was therefore of a differential and somewhat local 

 character in this region, though it appears to have been nearly contempora- 

 neous with important movements in far distant regions, notably that which 

 preceded the coal-forming- era in the Appalachians, and its effects were 

 probably felt over a very large portion of the North American Continent. 



The most visible results of this movement in the Colorado Island are 

 seen in the extensive erosion of Lower Paleozoic beds in Canyon City Bay, 

 from whose present position it would seem probable that the water connec- 

 tion with South. Park was interrupted and a land connection between the 

 main Colorado Island and its Wet Mountain extension was reestablished. 



mon xxvii 2 



