100 THE VERMILION IRON -BEARING DISTRICT. 



Unconformit}^ Greatest erosion interval in American geologj'. 



lUpper series. 

 IKeewatin group (possibl}' Huronian)]Van Hise's break. 



Archean 



Ontarian system Unconformity (?). 



[Coutchiehing group. 

 Irruptive unconformity. 

 Laurentian system. 



Lower series. 



WiNCHELL, N. H. Record of field observations in 1888 and 1889: Eighteenth 

 Ann. Rept. Geol. and Nat. Hist. Survey of Minn., for 1889, 1891, pp. 7-47. 



N. H. Winchell in 1891 g-ives numerous additional field observations. 

 The relations of the jaspilite, argillite, and green schist are considered, and 

 the arg-illite at least is reg'arded as a sedimentary rock (p. 9). In the Stuntz 

 conglomerate is found a large bowlder which contains ■ pebbles of chalce- 

 donic quartz and quartzose felsite — these contain pebbles of vitreous 

 quartz — and pebbles resembling the porphyrel at Kekekebic [Cacaquabic] 

 Lake (p. 31). A study of the ore formation leads to the conclusion that — 



All three of the known agencies for rock forming were intermittently' at work 

 and concerned in the formation of the iron ore, viz: Eruption to afford the basic 

 eruptive material; sedimentation, to arrange it (in the main), and chemical precipita- 

 tion in the same water, to give the pui'e hematite and chalcedonic silica [p. 42]. 



The following facts are given as evidence that the Grreat gabbro of the 

 Cupriferous formation lies below the Animikie slates and that the Kewee- 

 nawan includes both the Animikie slate and the Huronian (Potsdam) 

 quartzite. 



^■' The most important and significant fact that bears on the stratigraphical^position 

 of the gabbro, respecting its relation to the Animike black slates, is its occurrence 

 along a wide extent, reaching from Guniiint Lake as far southwestward as to the 

 railroad crossing at Mallmann's (at least), next to and immediately south either of the 

 gneiss of the Giant's range or of the "greenstones" of the Kawishiwin, without the 

 appearance of any of the black slates between them. There is an appearance of 

 quartzite, with olivine grains and with magnitite, geographically between the gneiss 

 and the gabbro, the same being unquestionably the Pewabic quartzite seen near 

 Gunflint Lake. This quartzite is sometimes impure and limonitic, and seems to be 

 the chief iron horizon of the Mesabi range. This near conjunction (which is some- 

 times apparently an exact contact) of the gabbro with the gneiss, and the absence of 

 the Animikie proper between them, has been supposed to be due to a local overlap 

 of the gabbro beyond the strike of the Animike, covering it from sight, the idea 



