42 HISTORICAL GEOLOGY 



ous rocks; and more or less schist, gneiss, quartzite, marble, and 

 some iron ore, representing all the common types of sedimentary 

 rocks in a highly metamorphosed condition. In the Lake Superior 

 district this series is called the Kewatin, and in eastern Canada 

 and the Adirondacks the Grenville. 



The plutonic series consists of tremendous masses of deep- 

 seated igneous rocks which are mostly red to gray granites, often 

 of different ages, and at times with more basic syenitic to even 

 gabbroic facies. A most important feature of this series, called 

 the Laurentian in the Lake Superior district and in eastern Canada, 

 is the fact that it is invariably intrusive into the first or lava- 

 sedimentary series. Thus as regards actual position in the earth's 

 crust, the Laurentian rocks generally he under the Kewatin or 

 the Grenville, but since the contact is clearly an intrusive one, 

 the law of superposition cannot here be applied for relative age 

 determination. 



The following tabular summary will serve to make clear the 

 subdivisions of the Archeozoic and its relation to the Proterozoic 

 in a portion of North America where the pre-Cambrian rocks 

 have been most carefully studied. 



Lake Superior District 



Paleozoic Cambrian 



Great Unconformity 

 [ 2. Keweenawan 

 (unconformity) 



f Upper (Animikian) 

 Proterozoic I (unconformity) 



(Algonkian) j 1. Huronian < Middle 



(unconformity). 

 { ( Lower 



Grea t unco nfo rmity 



[ 2. Laurentian granite 

 Archeozoic { (Intrusive into Kewatin) 

 [ 1. Kewatin series 



Correlation of Archean Rocks. — Because of the complete 

 absence of satisfactory methods of correlation, pre-Cambrian 

 rocks in one region cannot certainly be regarded as equivalent to 

 those in another region separated from it. Thus the Grenville 

 of eastern Canada cannot at present be certainly correlated with 



