374 C. W. HALL — KEEWATIN OP EASTERN AND CENTRAL MINNESOTA 



the interpretation the}^ afford of the genesis and stratigraphic relations 

 of the rocks under discussion. The regions named as affording similar 

 petrographic conditions are the Black hills of South Dakota, Penokee- 

 Gogebic iron range of Michigan-Wisconsin, the lake of the Woods, Rainy 

 lake, the International boundar}^ and the Mesabi iron range at Virginia; 

 which conditions point to these genetic states : 



1. A period of sedimentation took place, during which mediumly 

 coarse to fine silicious deposits were laid down over a large area and to 

 a great thickness, estimated not less than 20,000 feet. 



2. Following this came a period of volcanic activit}^ during which 

 enormous quantities of hornblende-biotite granite, originally augitic 

 acidic rocks, were poured out to the westward, probably contemporane- 

 ous with the granitic intrusions of the Mesabi range and Rainy lake. 

 These intrusions faded out into a series of minor bosses and dikes to the 

 eastward and in the central part of the area described. 



3. Farther away from this center of volcanic activity the schistose 

 condition of the sediments becomes less distinct until typical and slightl}^ 

 altered graj^wackes prevail. 



4. After an era of uplift and erosion, the entire area was subject to 

 further volcanic invasion, when extensive dikes of diabasic rocks were 

 forced into graywackes, schists, and granites alike; the southeastern 

 edge of the intruded rocks was forced down b}" the development of a 

 fault line and became covered with hundreds of feet of sandstone sedi- 

 ments, when the great Cambrian transgression of the sea took place. 



The age interpretation accepted for the rocks under discussion is that 

 of Spurr, who regards the rocks along the Saint Louis river as Keewatin, 

 basing his correlation on their relations, geogra})hic, structural, and lith- 

 ologic, to the Keewatin schists in the neighborhood of Virginia, on the 

 Mesabi iron range. 



The series along the Saint Louis river is shown to continue through 

 the Blackhoof valley and into that of the Kettle river as far as the ex- 

 posures west of Sturgeon lake as a series of finely crystalline hornblende 

 and hornblende-biotite schists. These Blackhoof and Kettle River schists 

 are therefore held to be of Keewatin age. 



The rocks stretching southwestward from Sturgeon lake across Snake 

 and Rum rivers into central Minnesota are, on account of their geographic 

 continuation and lithologic and structural habit, assumed to be a con- 

 tinuation of the same schists as were followed from the Saint Louis into 

 the Kettle River valley — that is, Keewatin. 



If the.investigations and opinions herein set forth prove in their gen- 

 eral features to be correct, the eastern and central portions of Minnesota 

 must be mapped as Algonkian rather than Archean, as has hitherto been 

 done. 



