﻿Iron Ores of the Lake Superior region. 261 



ginous rocks of the Menominee and Marquette regions, in Wis- 

 consin and Michigan ; and those of the Vermilion Lake region, 

 and thence northeastward, in Minnesota. This classification of 

 formations has no chronological significance; but the separa- 

 tion is a convenient one in the present connection, since the 

 processes of alteration have been carried furthest among the 

 folded rocks. Indeed, we find that the amount of change has 

 been somewhat in proportion to the degree of disturbance that 

 the rocks have undergone. Beginning with the flat-lying 

 Animike series, we find the largest amount of unaltered ma- 

 terial ; next in order come the highly tilted but unfolded Peno- 

 kee rocks; then the moderately folded Menominee and Mar- 

 quette formations ; and, finally, the closely folded rocks of the 

 Vermilion Lake district. It is not to be supposed, however, 

 that the series of changes is only made out by putting together 

 the rocks from these separate districts, the alterations for one 

 district beginning where those of another leave off. On the 

 contrary, we find in the flat-lying Animike nearly all the 

 changes, only less thoroughly carried out. In the Penokee 

 series the changes have been more general and are pushed a 

 little further, but some of the original material is still remain- 

 ing. Similarly among the folded rocks we find the changes 

 pushed to their extreme, there being but mere remnants of the 

 unchanged material. Each of these four series of rocks we 

 may now consider in order. 



The northern edge of the so-called Animike series, whose beds 

 range in position from approximate horizon tality to a southern 

 dip rarely exceeding ten degrees, runs from the northern 

 side of Thunder Bay, Canada, west and south to the national 

 boundary line, in the vicinity of North and Q-unflint lakes. 

 Thence it extends into Minnesota but a short distance before 

 it is overlapped and the Animike buried from sight, by an 

 immense spread of the coarse olivine-gabbro which lies at the 

 base of the Keweenawan Group in that region.* Sixty miles 

 farther southwest, in the vicinity of the so-called Mesabi Range, 

 Minn., the northern edge of the Animike reappears, remaining 

 uncovered by any newer formation, other than the drift, at 

 least as far west as the vicinity of the Mississippi River, an 

 additional distance of over eighty miles. West of the new rail- 

 way to Vermilion Lake, however, the rocks are badly con- 

 cealed by the glacial drift, and this portion of the Animike is 

 but little known ; but the remainder of its northern edge, along 

 a total distance of over eighty miles, has been examined at a 



* The existence of this most interesting overlap, announced here for the first 

 time, has been established mainly by the researches of Messrs. "W. jS\ Merriam 

 and W. M. Chauvenet of the U. S. Geological Survey. 



