78 GEOLOGICAL SURVEY OF WISCONSIN. 



" IV. In Micliigan and in all regions where magnetic iron ore is 

 found, mucli reliance in exploring is placed upon magnetic surveys. 

 Valuable mines have been discovered by noting the abnormal deflec- 

 tions of a delicate magnetic needle in passing along at right angles 

 to the trend of formations. This method often succeeds, when the 

 dip compass fails. Although magnetic surveys have not been made 

 in the region under consideration, lineal surveys have, and the varia- 

 tion at several points of each section recorded upon the township plats. 



" In the townships through which the Huronian belt would be ex- 

 pected to pass, the difference between the maximum and minimum 

 deflection to the east is much greater than in townships known to be 

 distant from magnetic influences. 



" For instance, township 37, range 20 west, the difference is 5° 39', 

 and township 36, range 20 west, the difference is 5° 30'. ISTumerous 

 other instances might be mentioned. The fluctuations of the needle 

 from a fixed point under ordinary circumstances, is not usually over 

 one to two degrees." 



It is only necessary to add here that the explorations in the Peno- 

 kee range for the discovery of iron ore would, in the judgment of 

 the undersigned, be more likely to prove successful, if they were 

 confined to boring with the diamond drill into the northern brow of 

 the mountain at right angles to the dip of the rocks of which the 

 formation is composed. The better ores are softer; therefore have 

 undergone more denudation than the harder ores ; consequently the 

 former are buried under surface debris, whilst the latter are exhibited 

 in outcrop. By traversing the magnetic schists with the diamond drill, 

 the quality of the borings can be constantly tested, and the true hori- 

 zon of valuable ores can be accurately ascertained. 



The long journeys down the Chippewa river, from the crossing of the 

 Central railroad to Eau Claire, up the "Wisconsin river from "Wausau 

 to the mouth of the Pelican, up the Pelican to its source, and down 

 the "Wolf from a short distance above Post Lake to Shawano, were 

 fertile in negative results, but afforded too few data to determine with 

 any degree of accuracy, boundaries between the Huronian and the 

 Laurentian formations in the wild Archtean region of the state. Ap- 

 parently the Chippewa river traverses an area of Laurentian till it 

 touches the Potsdam sandstone in its southeastward course. A Hu- 

 ronian range is known to enter the state from the Michigan side, at 

 and above the mouth of Pine river on the Menomonee, and to extend 

 westward. At Post Lake dam, on the "Wolf river, highly crystalline 

 hornblende rocks were examined, which, with other indications led 

 to the conjecture, that the Menomonee Huronian range crosses in that 



