338 THE CRYSTAL FALLS IRON-BE AEINdDISTEICT. 



SECTION II. DBSCRIPTIOlSr-OF THE MAGNETIC ROCKS. 



The magnetic rock of the Lower Huronian series of the western por- 

 tion of the Marquette area, which is of special importance to notice, since 

 it forms one of the chief horizons of reference to which our work is tied, 

 is the Negaunee iron formation. It is finely exposed at the south end of 

 the Republic trough; but farther north has been greatly reduced in thick- 

 ness, or locally cut out altogether,^ by the Upper Marquette denudation, 

 and, where present at all, is usually drift covered. 



This rock often possesses a very distinct banding, caused by the alter- 

 nation of layers, in which one of the constituent minerals predominates 

 over the others, sometimes, indeed, to their total exclusion. In the lower 

 part of the formation, quartz and griinerite constitute the bulk of the 

 rock, with magnetite scattered soniewhat indiscriminately through them. 

 Higher up, the magnetite and quartz relatively increase, until near the top, 

 but below the jasper, the griinerite goes out almost entirely, and the rock 

 consists of quartz bands, heavily charged with magnetite, in alternation with 

 bands of nearly pure magnetite. In the Negaunee formation, as exposed 

 at Republic, the magnetite therefore occurs concentrated in some of the 

 parallel bands and disseminated through the others. 



In the same district there is another much less prominent locus of mag- 

 netite at and near the base of the Upper Marquette or "hanging-wall" 

 quartzite. Along the strike of this zone, which is of small thickness, the 

 distribution of magnetite is very irregular; and for this and the additional 

 reason that when the magnetic portion of the Negaunee formation comes 

 up to it the disturbances which it produces can not be discriminated from 

 those produced by the latter, the position of the plane usually can not be 

 inferred. 



In the Menominee district and its extensions there are two horizons in 

 the lower series, characterized by the presence of magnetite. The lower 

 of these is not known to outcrop, but it occurs somewhere near the junc- 

 tion of the dolomite and the underlying quartzite. The magnetic disturb- 

 ances due to this formation are feeble, but they are quite persistent in the 

 Felch Mountain area, and have thrown some light on the geological structure. 



' The Marquette iron-bearing district of Michigan, by C. R. Van Hise and W. S. Bayley, with a 

 chapter on the Republic trough, by H. L. Smyth : Mon. U. S. Geol. Survey, No. XXVIII, 1897, pp. 531, 537, 



