390 THE VERMILION IRON-BEARING DISTRICT. 



aiForcl indisputable proof that the gabbro is younger than the Grunflint 

 formation. 



Relations to basic dikes. — The Gunflint beds have been cut by dikes 

 and sills of basalt similar to dikes which cut the Duluth gabbro. 



THICKNgSS. 



The Gunflint formation is shown on the map in the atlas as feathering 

 out in sec. 33, T. 65 N., R. 5 W., near the end of Paul Lake. East of this 

 point it widens very much and reaches its maximum width in sees. 23 and 

 26, T. 65 N., R. 4 W. East of these sections it again narrows down. Where 

 it is widest the beds have been somewhat crumpled locally by great intru- 

 sive sills. Gi'ant" has estimated the maximum thickness to be about 825 

 feet, calculated on an average dip of 10° S., but states that this estimate is 

 possibly too great. 



Irving and Van Hise estimated the thickness of the iron-bearing mem- 

 bers in the Penokee district of Wisconsin and Michigan to be from 800 to 

 1,000 feet.'' This seems to be, however, very close to the true thickness 

 for the Gunflint iron-bearing beds, as shown by comparison with correla- 

 tive beds in other districts. Thus the Biwabik formation of the Mesabi 

 district, with which this is correlative, has been estimated by Leith " to 

 vary from 200 to 2,000 feet in thickness, and to have an average thickness 

 of about 1,000 feet. 



SECTIOX II.— ROTE SLATE. 



The sediments constituting this formation lie immediately above the 

 Gunflint formation and have been called the Rove slate from Rove Lake, 

 a lake situated just north of the international boundary and east and out- 

 side of the Vermilion district, and lying in a large area underlain by these 

 slates in typical development. Slates occupying the same stratigraphic 

 position and possessing the same general characters occn.r in the Mesabi 

 district and are there known as the Virginia formation. 



«Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 471. 

 ''Mon, U. S. Geol. Survey Vol. XIX, 1892, p. 189. 



"The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey Vol. 

 XLIII, 1903, p. 166. 



