530 THE ^MARQUETTE IRONBEARIJfG DISTRICT. 



phases to be made up mainly of (piartz and griinerite, or of quartz and 

 iron oxides, or of griinerite and iron oxides. The iron oxides, too, may be 

 either magnetite or liematite. These mineral constituents are arranged 

 iu very distinct narrow bands whicli are parallel to the upper and lo\A-er 

 bounding surfaces of the rock. The bands ai'e not wholly regular, nor 

 are the\- continuous for great distances. They thicken and thin, taper out, 

 and break joint. It is certain that none of the minerals which now make 

 up the rock are original, and that the parallel banded structm'e signifies that 

 the processes of metamorphism through which it has reached its present 

 constitution were controlled by a primary bedded structure. 



The variations in external appearance produced by these considerable 

 variations iu composition are great. These variations are not whollj- irreg- 

 ular, and it is possible to distinguish in the different phases a definite 

 distribution through the iron-bearing member, which holds good within the 

 limits of tlu> Republic area. The lower portion of the formation is, on 

 the M'liole, characterized by the presence of griinerite and gray or dark- 

 colored quartz with magnetite, while the higher portion is chai-acterized 

 by the almost conq)lete absence of griinerite and by the presence of 

 speculiu- hematite and red quartz or jasper, which owes its color to the 

 intimate mixture of the little particles of hematite with the quartz. The 

 study of the western portion of the Marquette area alone would probably 

 justify, on the basis of difference in composition and external a]q)earance, a 

 division of the ii'on-bearing member into two distinct formations, a lower 

 griincrite-magnetitc-schist mend)er and an upper specular jasper member. 



In the Republic area magnetite also increases in amount in going from 

 lower to higher horizons, wliile griinerite decreases, so that just beneath the 

 specular jasper the iron-l)earing mend)er is mainly made up of bands of 

 exceedingly fine grained magnetite alternating with bands of dark to black 

 quartz, the color of which is due to the presence of a large amount of 

 included magnetite. 



Under the microscope the chief interest centers in the question of the 

 nature of the quartz, whether it is partly or wholly of fragmental origin. 



In the study of the slides no evidence has yet been found that any of 

 the quartz is fragmental. Here and there traces are seen of an original 



