106 THE VERMILION IRON-BEARING DISTRICT. 



the Lower Vermilion series being placed in the Lower Huronian and the 

 Animikie being placed in the Upper Huronian. 



The Lower Huronian is largely crystalline; the LTpper Huronian semi- 

 crystalline. Locally, along axes of intense plication, both the Lower 

 Huronian and Upper Huronian have been transformed into completely 

 crystalline schists. 



WiNCHELL, N. H. The crystalline rocks — some preliminary considerations as 

 to their structure and origin: Twentieth Ann. Rept. Geol. and Nat. Hist. Survey 

 Minn., for 1891, 1893, pp. 1-28. 



[n this article Professor Winchell gives the following as the descend- 

 ing siiccession of strata in northeastern Minnesota. This determination of 

 the succession represents, according to him, the consensus of opinion of 

 several geologists who have given special attention to the field evidences. 

 From this statement must, however, be excepted the Great gabbro horizon, 

 No. 3, as by some it is joresumed to have preceded, and not to have 

 followed, the Pewabic quartzite. 



1. Keweenawan or Nipigon series, unconformably beneath rocks 

 bearing the "Dikellocephalus" fauna, and consisting of fragmental and 

 eruptive beds, the upper portions behig almost entirely red sandstones. 



2. Alternating beds of eruptive sheets and fragmental rocks. The 

 fragmentals are thin-bedded slates, actinolite-schists, magnetitic jaspers, 

 cherts, and quartzites. The sheets are ordinary eruptives or pyroclastics. 



3 Lnmense qiTantities of true gabbro, often bearing titaniferous mag- 

 netite, are associated with contemporaneous felsites, quartz-porphyries, 

 and red granites. This gabbro includes several masses of the next older 

 strata, particularly the Pewabic quartzite. 



4. The Animikie. This series is characterized by a great quartzite 

 associated with the iron ores and cherts. The quartzite (Pewabic) lies 

 unconformably on all the older rocks. It is often conglomeratic, bearing 

 debris of the underlying formations. Within it is mingled volcanic tuffs 

 from contemporaneous eruptions. The Pewabic quartzite includes that of 

 Pokegama Falls, on the Mississippi, and of Pipestone Coiinty. In the 

 vicinity of contemporaneous volcanic disturbances its grain is fine, like 

 jasper, and sometimes it lias acquired a dense crystalline structure from 

 contact with the gabbro. 



5. The Keewatin. This is a volcanic series of great thickness, com- 

 posed mainly of volcanic tufi"s, presenting evidence of aqueous sedimen- 



