300 THE MARQUETTE IRON-BEARING DISTRICT. 



upward, Init how much of these exposures belongs with the lower forma- 

 tions it is, as has been said, impossible to state If the slate included with 

 the Wewe slate is correctly placed, above this is a breadth of quartzite of 

 1,000 feet, which, witli an avei'age dip of G5°, gives a thickness of about 

 900 feet. 



INTERESTING LOCALITIES. 



M.chigamme area. — Beginning at tlie northwest, there are numerous expos- 

 ures of quartzite just south of the Archean from the west end of the area 

 mapped for 4 miles to the east, that is, to sec. 21, T. 48 N., R 30 W. (Atlas 

 Sheet V). The rocks here adjacent to the granite are at places feldspathic 

 quartzites, but the ordinary phases are vitreous quartzites, which in some 

 cases are sericitic, in others cherty. 



North of the Michigamme mine the quartzite is found directly in con- 

 tact with a wall of the granite and hanging. upon it. The rock is here a 

 coarse, feldspathic, gra}^ or green quartzite. Tlu-ee feet from the contact 

 is a layer of conglomerate 10 to 15 inches in thickness, which contains 

 pebbles of quartz, the larger of which are coarsely crystalline, but the 

 smaller of which are granulated. In its upper parts the Ajibik quartzite 

 is interlaminated with the Slamo slate, the layers of quartzite between the 

 layers of slate being 1 to 6 inches in thickness 



In thin section the quartzite at the base of the formation has a finely 

 crystalline, sericitic, kaolinic, and quartzose background, Avhich contains 

 simple and complex grains of quartz, from those of small to those of 

 large size. Numerous flakes of biotite and blades of chloritoid are pres- 

 ent. All of the quartz grains show undulatory extinction or fracturing 

 At high horizons, between the grains of the quartzite there is abundant 

 chlorite and garnet, the outlines of the two minerals having a similar 

 appearance. Certain areas consist in part of chlorite and in part of garnet. 

 The chlorite is apparently an alteration product of the garnet, the latter 

 being the original mineral which developed within the rock. In the 

 layers of quartzite interstratified Avith the slate the dynamic effects are less 

 marked than elsewhere in the formation of this part of the area, and here 

 the quartz grains frequently show cores and distinct enlargements. The 

 matrix in which the quartz grains are set is similar to that in the quartzite 



