SUMMARIES OF PRE-CAMBRIAN LITERATURE OF 
NORTH AMERICA 
EDWARD STEIDTMANN 
University of Wisconsin 
VI. THE CORDILLERA OF THE UNITED STATES 
The most notable advances in the study of the pre-Cambrian 
of the Cordillera are the finding of pre-Cambrian sediments 
unconformably below the Lower Cambrian of the southern Sierra 
Nevada of California by Knopf and Kirk; additions to our knowl- 
edge of the extent and composition of the belt series; and the 
epoch-making investigations of the life-record of the Beltian by 
Walcott. 
Bastin and Hill' report that the principal rocks underlying 
Gilpin County, Colorado, and adjacent-parts of Clear Creek and 
Boulder counties are of pre-Cambrian age. The Idaho Springs 
formation, a quartz biotite schist of sedimentary origin, is really 
the most important. Sedimentary origin is inferred from the 
highly aluminous composition of certain phases, apparent bedding, 
highly quartzose and certain apparently conglomeratic phases, 
lime silicate phases probably representing metamorphosed lime- 
stone, and the lack of evidence of intrusive relations. 
Other pre-Cambrian rocks of the area include stocks of granite 
gneiss, quartz diorite, and hornblendite, and the younger Silver 
Plume granite. Granite pegmatites of various ages are abundant. 
Blackwelder and Crooks? state that the Medicine Bow range, 
west of Laramie, Wyoming, contains one of the most varied sec- 
tions in the western United States. They include the basal schist 
gneiss complex above which are more than 25,000 feet of slightly 
tk. S. Bastin and James M. Hill, ‘‘Economic Geology of Gilpin County and 
Adjacent Parts of Clear Creek and Boulder Counties, Colorado,” U.S. Geol. Surv., 
Prof. Paper 94 (1917), 379 pp-, 23 pls., 79 figs. 
2 E. Blackwelder and H. F. Crooks, ‘‘Pre-Cambrian Rocks in the Medicine Bow 
Mountains of Wyoming,” Geol. Soc. Am. Bull., Vol. X XTX (1918), pp. 97-08. 
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