PRE-CAMBRIAN FORMATIONS. 13 



class of nicks. The distinctly sedimentary rocks arc, however, not over 

 1,000 feet in thickness, and hence they occupy necessarilj but a very small 

 proportion of the total pre-Cambrian area represented on the map. It has, 

 therefore, been judged besl to preserve the Archean color, in accordance 

 with existing practice, for this area, and it will be referred to under this 

 term throughout the text; at the same time it is freely admitted as possible 

 that future study may show thai a much larger portion of this area than at 

 present appears probable should properly be considered as Algonkian. 



As the geological history of a region must be primarily determined 

 by a study of the sedimentary beds exposed in it and of the organic 



remains found in those beds, and as thus tar no representatives of the 



Lower and Middle Cambrian strata, so abundantly developed in Utah, 

 Nevada, and British Columbia, have been found in the Rocky Mountain 

 region, it is only for the period commencing with Upper Cambrian time 

 that any attempt can be made at present to trace out the details of its 

 geological development. Within this period the histories of earliesl and 

 latest phases are necessarily the most obscun — in the one case because the 

 sediments deposited in the earliest times have been more generally covered 

 up and buried beneath the accumulations of later ages; in the other because 

 the latest sediments have had less time to consolidate into hard rock and 

 have been the first to be removed by the destructive agencies that have 

 acted on the surface during the long modern period in which the region 

 has been exposed to subaerial erosion. 



CAMBRIAN LAND. 



The principal land areas of the Rocky .Mountain region which pro- 

 jected above the waters in which Upper < iambrian sediments were deposited 

 were the Colorado and Sawatch islands. The latter included the present 

 Sawatch Range, and was possibly connected with other land masses to the 

 south, hut was entirely distinct from the Colorado Island and was separated 

 from it by a bay or strait, in which considerable extents of Upper ( Jambrian 

 sediments were deposited. To the east of the Colorado Island, covering 

 the present area of the Great Plains, was the great mediterranean sea, a 

 shallow ocean which stretched with few known interruptions to the present 

 summits of the Appalachian Mountains. 



