SOME PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 583 



of Colorado appear as isolated patches of pre-Cambrian schists and 

 gneisses cut by pegmatite and granite. 



Schrader'^ states that the principal topographic features of 

 Mohave County, Arizona, are barren, plainlike, detritus-filled 

 valleys 10 or 12 miles in width, separated by ranges of about equal 

 width. Their trend is a little west of north, and the general slope 

 is toward the southwest. 



The ridges consist mainly of pre-Cambrian granites, porphyries, 

 gneisses and schists cut by pre-Tertiary intrusives. The pre- 

 Cambrian probably was once covered or nearly covered by Paleozoic 

 sediments which are now almost completely eroded. 



Schrader^ finds that the pre-Cambrian granites, gneisses, and 

 schists constitute the basement complex on which later sediments 

 of Mohave County, Arizona, have been deposited. No pre- 

 Cambrian sediments have been recognized. The trend of the 

 schists is about N. 30° E. and their dip is either vertical or steeply 

 east-northeast. 



Smith^ believes that the older gneisses of the Sierra Nevada are 

 probably Archaean. 



Spurr'' states that the lowest formation in the Aspen district is 

 pre-Cambrian granite. 



Stone^ finds that in the southeastern part of T. 5 N., R. 2 W., of 

 Montana there is a flat area, underlain by Algonkian sediments of 

 unknown thickness, mostly red shales, including some green shale 

 and thin sandstone beds. They are believed to correspond to the 

 Spokane shale to the east. 



^ F. C. Schrader, "The Mineral Deposits of Mohave County, Arizona," Btdl. 340, 

 U.S. Geological Survey, 1907, pp. 53-95. 



^ F. C. Schrader, "Mineral Deposits of the Cerbat Range, Black Mountains, and 

 Grand Wash Cliffs, Mohave County, Arizona," Bull. 397, U.S. Geological Survey, 1909, 

 pp. 220. 



3 James Perrin Smith, "The Geological Record of California," Jour. GeoL, XVIII, 

 No. 3 (1910), 216-27. 



•'J. Edward Spurr, "Ore Deposition at Aspen, Colorado," Econ. GeoL, IV (1909), 

 301-20. 



5 R. W. Stone, "Geologic Relations of Ore Deposits in the Elkhorn Mountains, 

 Montana," Bull. 470, U.S. Geological Survey, 1911, pp. 75-98, i map. 



