MAGNETIC LINES. 157 



but with no contact exposed. Tlii'oughout its extent the hne of disturbance 

 is separated from the hne of outcrops of the Hemlock volcanics by a short 

 intervaL It is, however, always distinctly separated from them. 



Magnetic line E. — This magnetic line passes directly through the open pits 

 at the Hemlock. As the line is traced north from this point it passes just 

 west of an amygdaloidal lava one-half mile north of the mine. From this 

 point until it is lost in sec. 16, T. 45 N., R. 33 W., there is no evidence in 

 regard to the nature of the rock causing the attraction. Tracing the line 

 south from the Hemlock mine it is found to swing about 200 paces east of 

 the Michigan mine, near the north line of sec. 9, T. 45 N., R. 33 W. 

 A quarter of a mile farther south it swings back again, apparently in the 

 line of the continuation of the iron-bearing formation, which it follows for 

 one-half mile farther, where it is lost. The only place along this line where 

 it has been possible to determine the rock causing the attraction is at the 

 Hemlock mine. Here it was found that it is not the ore formation proper 

 which is magnetic, but that it is the foot wall. This is a magnetitic slate, 

 about 42 feet in thickness, as shown by the diamond-drill borings. 



The above are the only lines of maximum magnetic disturbance of 

 this part of the Crystal Falls district which it has been found possible to 

 connect in any way closely with the iron-bearing rocks. A large number 

 of lines of disturbance, however, were traced within the limits of the 

 Hemlock formation, but on accoxmt of their slight economic importance 

 they are not inserted on the map. In these cases the influence on the 

 needle is evidently exerted by the magnetite of the lavas and pyroclastics, 

 and in proof of this the lines can very commonly be connected with 

 exposures of the various volcanic rocks. It is of interest to note that the 

 trend of the lines in the volcanics invariably agrees with that of the tuff 

 beds, and with the general strike of the formations of the district, and the 

 reader is reminded of the suggestion already offered (p. 1 34) that they may 

 be caused by magnetite accumulated by secondary processes, especially 

 active in the tuff beds and scoriaceous portions of the lava flows. 



THICKKBSS. 



Since the Upper Huronian sediments cover a broad area, their thick- 

 ness must be very considerable. Owing, however, to the scarcity of 

 exposui'es, it is impossible to give even an approximate estimate. 



