THE NORTHEASTBEN AEEA. 455 



B line must, therefore, be older than any member of the Upper Marquette 

 series. The Negaunee iron formation, represented in the A line, dips west, 

 while the rock of the B line dips east. They are both older than the basal 

 member of the Upper Marquette series, and are both younger than the 

 Archean. They are both strongly and persistently magnetic. For 8 or 1 

 miles they run parallel to each other less than half a mile apart. Their 

 broad structural relations to the Archean basement of the region are pre- 

 cisely similar. Therefore, although the rock that gives rise to the B line 

 has never yet been seen, it may be concluded with the utmost confidence 

 that it is the Negaunee iron formation, and that the A and B lines represent 

 this rock brought up in the two limbs of a narrow and probably deep 

 synclinal fold. 



This conclusion carries the Negaunee iron formation 3i miles farther 

 to the west, and in the northeast part of T. 45 N., R. 31 W., leaves a gap of 

 but 1^ miles between the Lower Marquette and the Menominee series. 



Here, between the B and C lines, is precisely the same situation as 

 between the A and B. One magnetic rock, represented by the B line, dips 

 west; the other, the Groveland formation, represented by the C line, dips 

 east. Between them no magnetic disturbances can be found. The area 

 between them must have a synclinal structure, and if they are not one and 

 the same formation each must undergo an extremely rapid and precisely 

 similar change in lithological character (namely, the loss of magnetite) in 

 a very short distance and be represented on the opposite side of the syn- 

 clinal fold by a nonmagnetic formation. Each of these rocks is persistently 

 magnetic in the direction of the strike for great distances. That each should 

 independently lose its magnetite in the direction of the dip in this particular 

 locality is very improbable. And, therefore, the grounds for the conclusion 

 that the B and C lines represent one and the same formation are quite as 

 firm as those upon which rests the conclusion that the A and B lines repre- 

 sent the same, formation. 



The greater portion of the Northeastern area is without outcrops, yet 

 through the structural and lithological results of the magnetic work we are 

 able to bridge over the gap and to show with a high degree of probability 

 that the Negaunee iron formation of the Marquette range is identical with 

 the Grroveland iron formation of the Felch Mountain range. Further, 

 when we recall the differentiation of the Groveland formation in the 



