120 C. R. Van Ilise — Iron Ores of Michigan. 



southwest end of this trough, where the Republic mine is 

 located, has a pitch to the northwest of 40° or 50°. At the 

 west end of the Marquette district proper the mines, such as 

 the Michigamme, Spurr, Champion, etc., are upon the sides of 

 a great synclinal, which, however, is probably a synclinorium. 

 In the great mining center of T. 47 N., R. 27 W., already 

 referred to, where are the towns of Ishpeming and Negaunee, 

 the formation has been folded into a series of rolls, which have 

 variable pitches, as a consequence of which there are here 

 several iron ranges approximately parallel. The mines, as in 

 the more simply folded part of the district, are frequently at 

 the sides of the synclinals, which, as already said, are gene- 

 rally flanked by diorite ridges. "Whether the upwelling of the 

 eruptives was the cause of the folding, or whether as the fold- 

 ing occurred fracture took place in the brittle ore-formation of 

 which the intruding rock took advantage, it is yet too early to 

 discuss. 



Classification of Lower Marquette ore- deposits. — The known 

 ore-bodies, with reference to their position, may be divided 

 into the following classes : (1) deposits at the contact of the 

 quartzite-conglomerate and the ore-bearing formation ; (2) 

 deposits resting upon soap-rock which grades into massive 

 diorite ; (3) deposits resting upon dikes of soap-rock which 

 follow along or cut across the ore-bearing formation ; (4) de- 

 posits interbedded in the jasper or chert. (See fig. 1, p. 123, 

 generalized section of ore-formation in which ore in its more 

 important relations is represented by cross-hatching.) 



(i.) Deposits at the contact of the quartzite-conglomerate 

 and the ore-hearing formation. — The ores occupying this posi- 

 tion are generally hard and either specular or magnetic. It is 

 to be remembered that the adjacent rock of the underlying 

 ore-bearing formation is usually banded ore and jasper, although 

 occasionally it is magnetite-actinolite schist. Because of the 

 constant association of the hard ores with the red form of 

 silica this material is sometimes called " specular jasper:" One 

 of the largest known deposits of the first class is that at Re- 

 public, and this locality, because of its magnificent exposures, 

 is particularly favorable for study. While the main bodies of 

 ore occur at the contact horizon, the mine maps show a con- 

 stant tendency to form offshoots, a part of the ore following 

 the banding of the jasper formation, which dips at a steeper 

 inclination than the contact plane. (Fig- 2, p. 123). A body 

 may continue in considerable force for some depth, but when it 

 gets far from the contact plane, it is apt to die out. At and below 

 the place where the branch strikes off the main deposit may 

 become somewhat narrower, but in passing downward it often 

 gains its full magnitude, and then a second shoot may start 

 back from the contact plane into the underlying jasper. 



