344 C. W. HALL KEEWATIN OF EASTERN AND CENTRAL MINNESOTA 



Page 



The granites 3b7 



The diabases 367 



The gabbro '. 368 



Veins and veinstuffs 368 



Age of the series 369 



Earlier views 369 



The present view 371 



Summary 373 



Explanation of plates 375 



Introduction 



Within Minnesota from the city limits of Duluth soutliwestward be- 

 yond the geographical center of the state, which is lake Mille Lacs, lies 

 a series of schistose rocks. To the southeast the}^ disappear beneath the 

 Cambrian sandstones, and the northvrest boundary is entirely concealed 

 by the Glacial drift. Their existence in a few localities has been known 

 since the earliest geological explorations of the region. They were first 

 recognized as Archean ; then as Animikie — that is, U})per Huronian ; and 

 lastly, in 1894, the northernmost exposures were pronounced by Spurr 

 to be Keewatin — that is. Lower Huronian. 



The pre-Cambrian age of the series, which has rarely been doubted, is 

 proved by the presence at two or more places of nearly horizontal sand- 

 stones in such relation to the rocks in question as to establish their sub- 

 sequent age. Again, at Short Line park, near Duluth, the series under 

 discussion is found to lie in undetermined thickness beneath the diabasQ 

 flows and associated rocks of the Keweenawan * Finally, since dikes 

 of granite are found breaking up the intruded schists within areas re- 

 garded as belonging to the series discussed, the post-Archean age of the 

 granites is with confidence asserted. 



Geographic Subdivisions 



Along the Saint Louis river where it constitutes the interstate boundary 

 a belt of slates and schists emerges from beneath the Keweenawan to 

 extend in almost continuous exposure up the river to Cloquet. The ex- 

 posures lie in force around Thomson and Carlton, where the rocks range 

 through every textural phase from a graywacke conglomerate to a roof- 

 ing slate of excellent quality. 



In the Blackhoof River valley the schists appear above the thin cover- 

 ing of drift and stretch southwestward to the Kettle River valley, where 



* Geological and Natural History Survey of Minnesota, final report, vol. iv, 1899, p. 567, 5G8. 



