CHAPTER III. 

 GEOLOGIC FORMATIONS UNDERLYING THE DRIFT. 



Archean, Lower and Upper Silmian, Devouian, and Cretaceous forma- 

 tions succeed each other from east to west as the bed-rocks of the area of 

 Lake Agassiz (PI. XIV). 'l^hey will be briefly described here in this order, 

 which is that of their age and superposition, beginning with the oldest and 

 lowest. Throughout large portions of this region, including the whole 

 district di'ained by the Red River in Minnesota, the underlying rocks are 

 covered by the glacial drift, and afford no outcrops; but their character and 

 approximate boundaries on these tracts are inferred with much probability 

 from the nearest outcrops, from topographic features, from the bowlders 

 and other material of the drift, and from sections shown by deep wells 

 which pass through the drift to the rocks beneath. 



Intervening in stratigraphic order between the Archean and Silurian 

 systems are large areas of the Algonkian and Cambrian systems, as mapped 

 on PI. XIV for the country about the west part of Lake Superior; but the 

 Algonkian and Cambrian rocks probably have no outcrops on the Lake 

 Agassiz area. 



ARCHEAN FORMATIONS. 



On the east side of the south part of Lake Agassiz a belt of Archean 

 rocks extends from the Minnesota River northeast and north, partly covered 

 w^est of Lake Superior by the Algonkian formations, through central and 

 northern Minnesota, where it widens into the m ain area of these rocks in 

 North America. This great Archean area stretches from Labrador and 

 the lower St. Lawrence southwest to Georgian Bay of Lake Huron, west 

 to Lakes Superior and Winnipeg", and thence northwest and north to the 

 Arctic Sea. Its western border was covered by Lake Agassiz from the 

 Lake of the Woods to the north end of Lake Winnipeg. 



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 MON XXV 5 



