PRE-CAMMl'IAX FORMATIONS. 11 



while tin- latter contain among their constituents a considerable proportion 

 of materia] which can be distinctly recognized as derived from the base- 

 ment of crystalline nicks, and the amount and relative coarseness of such 

 material vary with its distance from the ancient shore-line. 



According to the classification in vogue at the time the earlier geo- 

 logical maps of the Rocky Mountain region were made, the whole of this 

 older series of prevailingly crystalline rocks was mapped as Archean, 

 although, even in the earliest reconnaissances, various observers had recog- 

 nized that it included two or more distinct groups, in some of which a dis- 

 tinctly sedimentary nature and stratified structure could be distinguished. 



In later years, since the discovery in the Lake Superior region of 

 immense thicknesses of distinctly stratified clastic or fragmentary beds 

 older than the earliest Cambrian, yet more recent than, and as a rale 

 unconformable with, the underlying crystalline complex, the practice has 

 been adopted by the Survey of confining the term Archean to the oldest 

 crystalline rocks, and of grouping all fragmentary or clastic rocks which 

 are older than the Lowest fossiliferous Cambrian in a new system called 

 Algonkian. 



Although no systematic study has yet been made of tin.- greal areas of 

 more or less crystalline pre-Cambrian rocks in the Rocky Mountain region 

 with the view of differentiating the Algonkian from the true Archean rocks 

 that may occur in them, several small areas of Algonkian rock series have 

 been recognized which possess such distinctly sedimentary characteristics, 

 and are so situated with regard to Cambrian beds on the one hand and to 

 large bodies of rock hitherto classed ; i~ Archean on the other, that there 

 can be little doubl of the correctness of their assignment to this age. Such 

 are those of the Quartzite peaks, south of Silverton, and of the Uncom- 

 pahgre Canyon, south of Ourav. in the San Juan Mountains. 



In the Colorado or Front Range occurrences of distinctly fragmental 

 or clastic rocks, more or less completely inclosed in the granite-gneiss 

 complex, have been noted at various points, hut their relations with adjoin- 

 ing rocks have not yet been worked out over any large or connected areas. 



In the Pikes Peak region Mi*, ('ross' has described several bodies of 



Geologic Atlas of the United States, Pikes Peak folio, Washington, 1894, Explanatory text, 

 Algonkian period 



