420 THE MARQUETTE IllOX-liEAIIIXG DISTEICT. 



and the o-riinerite-uiag-iietite-.scliists, thus makiug the hxttei' rocks the lowest 

 horizou of this ore-1 )eanuo- formation. 



TUIf'KNBSS. 



The formation varies from a considerable thickness to disa]>pearance, 

 and, as in the previons cases, it is impossible to give an accurate estimate 

 of its thickness at any place. At some places it apparently has a surface 

 width of 600 feet, with a dij) varying somewhat, but perhaps averag-ing 60°. 

 This would indicate a maximum thickness of about 520 feet. 



INTERESTING LOCALITIES OF THE ISHPEMING FORMATION. 



Michigamme and Spurr. — Beginning at tlic nortliwest part of the district, there 

 are large and typical exposures of the Groodrich quartzite and Bijiki schist 

 from the Spurr mine to east of the Michigamme mine (Atlas Sheet V). At 

 these mines a considerable part of the magnetic ore apparently belongs to the 



Fig. 26. — Section abowing relationa of JaspUite, ore, conglomerate, and quartzite at Michigamme mine. 



basal horizon of the Islnjeming- formation. The ore deposits weld together 

 the unconformable Ishpeming and Negauuee formations. Tlie hanging 

 wall of the ore deposits is a conglomerate, the pebbles of which are mainly 

 from the underlying Negaunee jasper, and the matrix of which is mainly 

 magnetite. At the Spurr mine the pebbles are of the white, cherty rock; 

 at the j\Iichigamme mine they are of the red jasper, the larger ones 

 being- 6 to 8 inches in diameter. This suggests that the underlying Negau- 

 uee rock liad assumed its present form before it was broken up, and yielded 

 detritus to the Ishpeming fonnation. It is, however, possil)le that at 

 each ])lace sul)sequent changes have altered the upper part of the sili- 

 ceous ro(-k of the Negaunee formation and the overlying conglomerate in 

 a similar manner. At the Michigamme mine the conglomerate is in places 

 ,at least 20 feet thick, and in other places is absent, the quartzite directly 



