24 GEOLOGY OF THE DENVER BASIN. 



of Texas and Mexico and the Kootanie series of British Columbia, whose 

 representatives have for a long time been supposed to be wanting in the 

 Rocky Mountain region. The area of this assumed nondeposition lias been 

 somewhat circumscribed of late years by the discovery of Kootanie beds 

 in Montana, and more recently in the Black Hills region, and of Comanche 

 beds in New Mexico on the east front of the Rocky Mountains, not far 

 south of the Colorado boundaiy. The intermediate region, however, com- 

 prising the higher portion of the Rocky Mountain region of Colorado and 

 a considerable portion of the Great Plains opposite or to the east of this 

 region, appears, as in Jurassic time, to have been cut off from the access of 

 ocean waters, since no representative either of the marine Jura or of the 

 earlier Cretaceous or Neocomian beds has yet been found in it. 



From their fauna the Morrison beds are assumed to have been deposited 

 in fresh or lacustrine water, whereas the Dakota beds are distinctly marine, 

 and the character of the conglomerate at their base indicates that they were 

 deposited in an ocean that was slowly advancing over an area that had for 

 a long time been exposed to subaerial disintegration. 



The physical data from which the character of the movement may be 

 interred are as yet somewhat meager. It was assumed that the lake in 

 which the Morrison beds were deposited was separated from the ocean by 

 a harrier raised during the previous (Jurassic) movement along a line 

 extending eastward in the latitude of the Raton Hills and probably con- 

 nected with the old Paleozoic elevations of northern Texas, the Indian 

 Territory, and western Arkansas. A similar harrier may have extended 

 eastward in about the latitude of the northern boundary of Colorado. It 

 was assumed, further, that during the early Cretaceous movement these 

 barriers were broken and the region possibly slightly elevated, so that the 

 .Morrison Lake was drained ami its bottom remained above the ocean during 

 early Cretaceous time, exposed to subaerial disintegration and to some slight 

 erosion. At the (dose of the movement a general depression is supposed to 

 have set in, which continued to the middle of the Upper Cretaceous cycle 

 and during which the ocean waters filled the area formerly occupied by the 

 Morrison Lake. The evidence of the movement in this area is found in 

 the varying thickness of the Morrison formation from point to point. This 



