458 GEOLOGY 



respectively, geographic changes occurred, resulting in erosion 

 where sedimentation had been in progress. 



The source of the material for the sedimentary part of the first 

 of these systems was doubtless the exposed part of the Archean; 



Fig. 352. Map showing (in black) the position of the iron-producing areas 

 in the Lake Superior region. 1, Michipicoten district; 2, Kamamstquia 

 and Matawin district; 3, Steep Rock Lake and Attikokan district; 

 4, Vermilion district; 5, Mesabi district; 6, Penokee-Gogebic district; 

 7, 8, and 9, Marque tte, Crystal Falls, and Menominee districts. 



the source for the sedimentary part of the second system was the 

 exposed part of the Archean and the Lower Huronian systems, 

 and the source for the sedimentary part of the third system, the 

 exposed parts of all older formations. 



In places, the sedimentary rocks still remain in the condition 

 of conglomerate, sandstone, and shale, though more commonly the 

 sandstone has been changed to quartzite or quartz schist, and the 

 shale to slate or schist. Some of the igneous rock remains massive, 

 while some of it has been changed to schist. The rocks which aiv 

 least altered are, as a rule, those which have been least deformed, 

 and in places they are still nearly horizontal, as when first deposited. 

 The movements which deformed and altered the second system 



