THE NOETHEASTEEN AREA. 453 



limit of miuiiig development, though not of exploration, on the south side 

 of the Marquette synclinorium. Up to this point outcrojjs, j^roducing 

 mines, and old explorations are sufficiently abundant to permit the separate 

 formations to be traced and mapped with comparative ease, and to indicate 

 at least the larger structural features. 



At this northwestern end of the Republic trough the Lower Marquette 

 series makes an abrupt turn to the south, and may be followed for a mile 

 or more by occasional outcrops and test pits. The Negaunee iron formation 

 is persistently present beneath the Upper Marquette quartzite, and gives 

 rise to a A^ery strong and persistent line of magnetic atl action, which was 

 followed in our work for about 12 miles to the south and southeast into the 

 Northeastern area. For about 4 miles from the sharp turn at the mouth of 

 the Republic trough it runs nearly due south; afterwards it turns somewhat 

 to the east of south, and follows that course for about 6 miles, after which 

 it turns more and more toward the east, and finally, where we left it, its 

 course was only slightlj^ south of east. That this magnetic line is caused 

 by and marks the position of the Negaunee iron formation there can not be 

 the slightest doubt, for that rock outcrops in a few scattered localities, occurs 

 abundantly in the drift, and has been found in occasional test pits and drill 

 holes throughout this distance. The underlying quartzite outcrops beneath 

 the iron-bearing formation near the northern end of the line, but farther 

 south it is entirely covered by the drift, so far as the territory has been 

 examined. The overlying Upper Marquette rocks are also known to 

 be present just west of the Negaunee formation as far south as sec. 19, 

 T. 46 N., R. 30 W. 



The magnetic line which accompanies the Negaunee formation may be 

 called the A line. Taking into account the connected Republic trough and 

 its exposures of the Lower Marquette rocks, it is seen that the A line par- 

 tially surrounds a dome of the Archean crystallines, ai^d that in going 

 from the interior of this dome outward across the A line we pass from older 

 to younger rocks. The dip along the A line is, therefore, on the whole, 

 toward the west, although the obserA^ed dips at the few localities where 

 determinations have been made are either vertical or slightly inclined from 

 the vertical toward the east. The southern part of the A line, as far as it 

 has been ti-aced, passes through sees. 5, 8, 9, 15, and 16 of T. 45 N., R. 30 

 W. In section 5 it is just 5 miles east of the Groveland formation, Avhieh. 



