454 THE CEYSTAL PALLS IRON-BEAEING DISTRICT. 



as was shown in earlier chapters, is a magnetic rock occupying a definite 

 place in the Menominee succession, and is underlain by other typical 

 Menominee formations, and finally by the Archean. 



Between the A line and the magnetic line caused by the Grroveland 

 formation, which may be called the C line, is a third magnetic line, which 

 may be called the B line. This was traced parallel to the A line and less 

 than half a mile away, from near the south end of the latter to the north 

 end, and finally entirely round an elliptical area, closing again iipon itself 

 at the starting point, the perimeter of the ellipse being 25 miles in length. 

 Throughout this entire distance not a single outcrop could be discovered 

 along the B line. Within the inclosed area, however, in sees. 6 and 7, 

 T. 45 N., R. 30 W., and in sec. 19, T. 46 N., R. 30 W., several exposures 

 of granites and crystalline schists were found, which left no doubt that 

 the greater part of the area inclosed by the B line is occtipied by Archean 

 rocks of the same general character as those partially inclosed by the A 

 line on the east and entirely by the C line on the west. The area between 

 the A and B lines as far south as sec. 19, T. 46 N , R. 30 W., has been 

 proved to contain the basal member of the Upper Marquette series. The 

 southwestern quadrant of the B-line ellipse is nearly parallel to the C line 

 and only IJ miles away. 



The known facts with reference to the B line, then, are these : (1) It 

 represents a magnetic rock; (2) this magnetic rock completely encircles an 

 Archean core. It may further be inferred with practical certainty that this 

 formation, which carries such constant magnetic properties for 25 miles, 

 must be sedimentary. With regard to its structure, the foregoing con- 

 siderations would necessarily involve the conclusion that it dips away from 

 the Archean core on all sides, and this conclusion is fortified by the 

 unsymmetrical separation of the horizontal maxima on the magnetic cross 

 sections. It follows, therefore, that on the eastern side of the oval, where 

 the formation is parallel to the A line, it dips toward the east, and on the 

 western side, where it is parallel to the C line, it dips toward the west. 

 This conclusioii is further supported by the dips within the ellipse in the 

 outcropping Archean rocks that show structure. These all happen to lie 

 east of the major axis, and all dip toward the east. 



East of the B line, and between it and the A line, is found the basal 

 member of the Upper Marquette series. The rock which is manifest in the 



