24 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



MAGNETIC OBSERVATIONS. 



It has long been known that many rocks are possessed of decidedly 

 magnetic properties, due to the presence in them of varying quantities of 

 magnetic iron ore. By the mining engineers and prospectors tliis property 

 has been turned to a practical use in aiding in the location of iron mines 

 where the ore is of a magnetic kind. It is only in the past three decades 

 that this property has been used to any extent by geologists as an aid in 

 the interpretation of the structure of a region. So far as I can learn, the 

 best published account of its use thus is in Brooks's report on the iron- 

 bearing regions of Michigan.^ Conclusive proof of its geological value 

 was given in the mapping of the Penokee area, in 1876, by R. D. Irving 

 of the Wisconsin survey.^ That area extends for about 60 miles northeast- 

 southwest, and is on the average about 4 miles wide. For the eastern part 

 of the Wisconsin area the outcrops are few, and Irving located the iron 

 formation by magnetic work. Along that belt have been sunk shafts 

 belonging to various mines which have raised quantities of ore, and in no 

 case has a shaft sunk outside of the limit indicated by Irving come upon 

 paying ore. 



By means of the dip needle and solar compass, observations were taken 

 which enabled us to trace a curving magnetic formation and connect the 

 outcrops, which were separated by about 16 miles. The same bed was 

 further delimited, and the direction partly checked, by the occurrence, at 

 varying distances along this course, of outcrops of rocks of the imderlying 

 formation. 



Since the second part of this report contains an exhaustive article on 

 the methods and use of the magnetic needle,^ the subject is not further 

 treated here. The lines of maximum magnetic disturbance — or briefly, the 

 magnetic lines — are represented on the accompanying general map, PL III, 

 by blue lines marked with letters I) and E. 



' Magnetism of rocks and the iise of the magnetic needle in exploring for ore, hy T. B. Brooks. 

 Geol. Survey of Michigan, Vol. I, Part I, 1873, pp. 205-243. 



■^Geol. of the eastern Lake Superior district, by I?. T>. Irving. Geol. of Wisconsin, Aol. III,. 

 1880, i)p. 53-238. Atlas slu-ets, XI-XXVI. 



' See Part II, Chapter II, by H. L. Smyth, pp. 33(3-373. 



