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.HISTORICAL GEOLOGY 



Pre-Cambrian Eras 



In no connection is the saying, "All beginnings are difficult," more 

 true than in the study of the earliest chapters of the earth's history. 

 This is the case not only because the rocks which preserve the 

 record in a given region have been subjected to all the foldings and 

 metamorphisms which have affected all the subsequent rock forma- 

 tions of that region as well as earlier ones, but also because no fossils 

 have been found except near the close of the Pre-Cambrian, and these 

 are few and fragmentary. 



A brief classification of the Pre-Cambrian 1 of the Lake Superior re- 

 gion of North America is as follows : 



Proterozoic One or more series separated by unconformities to which 



(Greek, proteros y \ local names have been given since it has not been possible to 



early, and zoe> life) [ determine their equivalents in distant regions. 



Upper Proterozoic (Keweenawan) 



Unconformity 



Middle Proterozoic (Upper Huronian) 



Unconformity 



I Middle Huronian 

 Unconformity 

 Lower Huronian 

 Great Unconformity 



Proterozoic in the 

 Lake Superior region. 



Archtzozoic 

 (Greek, arche, begin- 

 ning, and zoe, life) 



(Laurentian) 



(Keewatin) 



Mainly light-colored (acid) granites, gneisses, 

 and schists. These are largely intrusive, but 

 some may represent the surface upon which 

 the Keewatin was laid down. 



Mainly dark-colored (basic) metamorphic 

 rocks, composed largely of metamorphic lava 

 flows and tuffs, with small amounts of meta- 

 morphic sediments. 



1 The United States Geological Survey includes all of the Pre-Cambrian rocks under the 

 term Proterozoic and uses Archaean for the Lower (Archaeozoic) and Algonkian for the Upper 

 Proterozoic. 



F. D. Adams considers that the Pre-Cambrian rocks have a threefold division which he 

 designates as Eo-Proterozoic (Archaeozoic), Meso-Proterozoic (Middle and Upper Huronian), 

 and Wo Proterozoic (Keweenawan), Proterozoic being used independent of any consideration 

 of the presence or absence of life. 



