HISTORICAL SKETCH—LAKE SUPERIOR REGION. II9 



ity may possibly be due in part to highlands of the Basement 

 Complex, which were not covered by the Lower Huronian sea 

 until the period was well advanced. Of the extent of the series 

 at the end of the erosion preceding Upper Huronian deposition, 

 little has been determined, since later erosions have undoubtedly 

 removed large areas of the series, and therefore its present dis- 

 tribution is not a safe guide to its distribution at the close of the 

 erosion interval referred to. 



The Upper Huronian. — At the close of the long period of 

 erosion which followed the Lower Huronian deposition, the 

 water once more advanced upon the Lake Superior region, and 

 the Upper Huronian series was deposited. 



Lithologically this series consists of conglomerates, quartzites, 

 graywackes, graywacke-slates, shales, mica-schists, ferruginous 

 slates, cherts, jaspers, ferruginous schists and igneous rocks, 

 including both lava flows and volcanic fragmentals, as well as 

 basic and acid intrusives. The series, as a whole, is very much 

 less crystalline than the Lower Huronian, although locally the 

 shales and graywackes have been transformed into mica-schists, 

 and even into gneisses. 



The Upper Huronian immediately about Lake Superior is 

 divisible into three formations, a lower slate, an iron-bearing for- 

 mation, and an upper slate, the basis of separation being that of 

 mechanical and non-mechanical detritus. The inferior formation 

 is mainly a quartzose slate or shale, but locally it passes into a 

 quartzite, while the basal horizon is frequently a conglomerate. 

 The nature of this conglomerate varies greatly, depending upon 

 the character of the underlying formation, which, in some areas, 

 is the Basement Complex, and in others the Lower Huronian. 

 In the first case the slates may rest upon the gneissoid granite, 

 upon the schists, or upon the junction of the two. The basal 

 conglomerate corresponds in its character, being a recomposed 

 granite or granite-conglomerate, a recomposed schist or schist 

 conglomerate, or finally a combination of the two. 



When the lowest member of the Upper Huronian rests upon 

 the Lower Huronian series, the underlying formation may be 



