CHAPTER IV. 



THE MICHIGAMME MOUNTAIN AND FENCE RIVER AREAS. 



By reference to the general map, PI. Ill, it will be seen that an oval- 

 shaped' Archean area, about 11 miles long from northwest to southeast and 

 having an extreme breadth of nearly 4 miles, runs through portions of 

 Ts 44, 45, and 46 N., Rs. 31 and 32 W. The country to be described in 

 the present chapter includes that portion of this Archean mass (together 

 with the younger rocks on its eastern border) which lies east of the line 

 between Ranges 31 and- 32 W., as well as the territory to the south in the 

 prolongation of the axial line, as far as the south line of T. 43 N., R. 31 W. 

 A gap about 6 miles broad not covered by our work intervenes between 

 this arbitrary southern boundary and the western termination of the Felch 

 Mountain work at Randville. 



In the northern portion of the area now under consideration (which 

 lies along and is twice crossed by the Fence River) the geological structure 

 is exceedingly simple, while in the southern portion, especially in the neigh- 

 borhood of Michigamme Mountain, it is rather complex. The boundary 

 between these two divisions falls in the neighborhood of the mouth of the 

 Fence River in sec. 22, T. 44 N., R. 31 W. It is therefore convenient 

 in what follows to refer to the northern portion as the Fence River area, 

 and to the southern as the Michigamme Mountain area. 



By referring to PI. HI, the broad geological structure of the whole 

 territory of which the above-mentioned Archean oval is the center is evi- 

 dent at a glance. It is an anticlinal dome, the core of which is Archean, 

 around which the younger Algonkian formations run in a series of concen- 

 tric rings, on all sides dipping outward from the inner nucleus. In the 

 Fence River area, on the eastern long side of the dome, the Algonkian 

 formations have a constant eastward dip, and are free from important 

 secondary folds. In the Michigamme Mountain area, however, which lies 

 in the prolongation of the main axis of the dome, these encircling forma- 



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