SUMMARY 373 



forced by the lithologic results already cited (ante, page 357), for the 

 Penokee range, lake of the Woods, Rainy lake, Black hills, Virginia, 

 Minnesota, and the personal observations of the writer along the northern 

 boundary of Minnesota, where bosses of granite, dikes of the same rock, 

 and diabase dikes break through the schists, which a few miles to the 

 eastward give place to clastic rocks either through the waning strength 

 of alteration processes or by superposition in identically the same man- 

 ner as seen in this region under review. 



The further proposition is proved : Much later than these events 

 another period of volcanic activity occurred, in which basic eruptivesin 

 the form of dikes of diabase porphyry were intruded into the uplifted 

 and eroded schists and acid eruptives, and a great fault line was devel- 

 oped which marked the line of weakness, defining the line of volcanic 

 vents, out of which poured the enormous lava flows of the Chengwatana 

 series of the Keweenawan.^ No trace of these dikes has yet been found 

 in the Cambrian. 



Summary 



Along the eastern border of Minnesota, and extending westward be- 

 yond the geographic center of the state, lies a belt of graywackes, schists, 

 and both acid and basic eruptives. Around Thomson, Carlton, and Clo- 

 quet the rocks are chiefly graywackes and graywacke slates. Clay slates, 

 carbonate schists, and diabase dikes are associated with them. South- 

 ward rocks occur which are plainly altered from the graywacke type 

 just named. As shown by a very continuous series of exposures, these 

 extend with practically no change in lithologic characters well into the 

 Kettle River valley. Their attitude is practically without change — that 

 is, they slope continually southward at an angle varying between 5 and 

 20 degrees until the district west of Sturgeon lake is reached. At this 

 point the rocks are hornblende and hornblende-biotite schists carrying- 

 minerals of contact significance and interbedded with an interesting 

 body of limestone which to a considerable extent is quartzose. 



To the west of Sturgeon lake the attitude of the rocks is changed. Pass- 

 ing to the Snake River valley, schists occur gradually broken by granite 

 dikes. From the Snake river westward to the Mississippi, granite be- 

 comes of growing importance until, west of the river, no known exposures 

 of schist occur, the rocks being wholly hornblende-biotite granites, with 

 which are associated dikes of diabase porphyry and bosses of biotitic 

 gabbro. 



The petrographic characters of the rocks are named. They correspond 

 precisely with the stratigraphic and structural characters just stated in 



* This Bulletin, ante, p. 327. 



