HISTORICAL SKETCH— LAKE SUPERIOR REGION. I I 7 



laterally, so that when one becomes familiar with them, the rocks 

 of the formation may invariably be recognized. Here are included 

 hematitic and magnetitic schists, cherts, jaspers, ferruginous 

 carbonates, and other forms. The formation always differs from 

 the limestone in carrying a very considerable amount of iron, and 

 it differs from the quartzite in being largely, and sometimes 

 wholly, a chemical or organic sediment, rather than a mechani- 

 cal one. 



The three members of the Lower Huronian are not often seen 

 in a single section. This may be due to lack of exposures, but 

 in some cases is undoubtedly due to the absence of one or more 

 of the formations themselves. 



In the Lower Huronian, basic eruptive rocks are abundant, 

 and locally cover considerable areas. Not infrequently acid 

 eruptives also occur. These eruptives include both contempor- 

 aneous volcanics and subsequent intrusives. If the Keewatin of 

 Lawson about Rainy Lake and the Lake of the Woods is Lower 

 Huronian, great granitic masses have been intruded into this 

 series northwest of Lake Superior. 



Equivalent to the Lower Huronian series of the north shore 

 of Lake Huron are placed the following iron-bearing districts : 

 Lower Vermillion, Lower Marquette, Felch Mountain, in large 

 part, Lower Menominee, the cherty limestone formation of the 

 Penokee district ; and probably the Kaministiquia series of 

 Ontario, and the Black River Falls series of Wisconsin. Whether 

 all of these detached basins were once connected by continuous 

 sediments is unknown, but probably they were. 



The fragmental material of the Lower Huronian was derived 

 from the Basement Complex. This fragmental formation is usu- 

 ally thin. This doubtless means that the advance of the sea 

 over the Lake Superior region was comparatively rapid. The 

 directions from which the Lower Huronion sea entered, and the 

 extent of its trangression, is at present unknown. By certain of 

 the Canadian geologists it is held that the structural break which 

 exists between the Basement Complex and the Lower Huronian, 

 south of Lake Superior and north of Lake Huron, does not exist 



