UPPER HURONIAN. 375 



miles to the west of the lake, which is a well-known feature of the inter- 

 national boundary route. Rocks with which these are correlated occur in 

 the'Mesabi district, and are there known as the Biwabik formation. 



DISTRIBUTION, EXPOSURES, AND TOPOGRAPHY. 



Distribution. — The iron formation has a wide distribution in the Mesabi 

 district, extending- through it from end to end. The Grunflint formation 

 can be looked upon as the eastern continuation of the iron-bearing Biwabik 

 formation of the Mesabi district. In the Vermilion district this iron forma- 

 tion has a restricted distribution. The area in which the rocks occur is in 

 places not more than about 200 paces wide, from this spreading out to a 

 width of nearly half a mile. Northeast of Paulson's mine, in sees. 21 and 22, 

 T. 65 N., R. 4 W., there is an east-west tongue of the Grunflint rocks project- 

 ing westward into Ely greenstone. About three-fourths of a mile east of 

 Paulson's mine, in sec. 27, T. 65 N., R. 4 W., where a gTeat north-south valley 

 cuts directly across the Grunflint formation, the narrow belt of iron formation 

 joins a wider area of the same rocks, which extends over the greater portion 

 of sees. 23 and 26, T. 65 N., R. 4 W. The Gunflint formation is widest in 

 these sections, its gi-eat width being due chiefly to the fact that a fairly wide 

 synclinal fold has here been stripped, leaving exposed an unusually large 

 area of the iron formation. East of these sectiotis, toward the international 

 boundary, the formation thins down rapidly. The northern boundary of 

 the Gunflint formation in this area is marked by the Knife Lake slates and 

 Ogishke conglomerate of the Lower Hui'onian series, and the Ely green- 

 stone and granite of Saganaga Lake of the Archean. This is the order in 

 which the Gunflint formation comes in contact with these rocks from west 

 to east. This is also the stratigraphic order, passing from tlie youngest on 

 the west to the oldest on the east, with the single exception of the granite 

 of Saganaga Lake, which intrudes the greenstone. 



The southern boundary of the iron-bearing formation over the greater 

 portion of the area in which it occurs is the northern edge of the Duluth 

 gabbro. From the SW. { of sec. 26, T. G5 N., R. 4 W., however, the direc- 

 tion of the southern boundary changes. From here it swings northeastward 

 and the Rove slates, which overlie it, begin to appear with a narrow edge 

 of a wedge widening to the east and separating the Gunflint formation from 

 the srabbro. 



