H. D. Irving — Is there a Huronian Group f 259 



the members of the successions in the two regions, that little 

 doubt can remain as to the equivalency of these members, each 

 to each. In each region we start with a great basement complex 

 of crystalline schists, gneiss and granite, upon whose immensely 

 denuded upper surface reposes a great quartzitic and slaty detri- 

 tal group, carrying sheets of chemically deposited iron and lime 

 carbonates. In each region this is succeeded in turn, but with 

 an intervening discordance, by the great Keweenaw series ; 

 while above this again follows the Potsdam sandstone. 



V. I will next ask your attention to the relations borne by 

 the so-called Animike series to the older schists and granites of 

 the region north of Lake Superior. These relations furnish 

 us with an excellent instance of an unconformity in which 

 the upper series has been tilted into an inclined position since 

 its original accumulation, but in which the amount of tilting 

 has not been great. In other words it is a case where the 

 conditions are not greatly different from those obtaining when 

 the upper formation above an unconformity has remained 

 horizontal. 



SJS, Sejyeyenah' 



■^z^^^^^r"^^---^-'- '■'•■'-' 5s Rfr^^^H^'^laS 





I 



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JKifel 



I *j3rwmzk,e Series 



Figs. ? and 8. — Generalized and partly ideal sections of the northeastern part 

 of Minnesota. Scale, 16 miles to the inch. 



The accompanying map and sections show* in a general 

 manner the geology of this region as known to date. Since 

 this map is in large measure based on hitherto unpublished 

 material, and since the geology of the area it represents has 

 been heretofore, so far at least as the interior is concerned, 

 largely conjectural, it is desirable that I should give here 

 somewhat more of an account of the structure of this region 

 than would otherwise be necessary. 



* The sections only are here given, on a greatly reduced scale. The map dis- 

 played to the Academy was a large colored one, drawn to a scale of four miles to 

 the inch, of all of that portion of the United States north of Lake Superior, and 

 east of the upper Mississippi ; showing the distributions of the Potsdam sand- 

 stone, of the Keweenaw series and of the great gabbro at its base, of the Animike 

 series, of the Vermillion Lake iron-bearing rocks and of the older crystalline 

 schists, with the great granite and gneiss areas whose contacts with the schists 

 appear to prove the former the newer rocks. This map, embodying the latest 

 geological and geographical information, is now in press for the Seventh Annual 

 Report of the U. S. Geological Survey. 



