570 R. C. ALLEN 



3. The iron-bearing member. — 'Like the other formations in the 

 Gwinn series the iron-bearing member varies markedly in thickness 

 but is nevertheless persistent, occupying a constant and definite 

 stratigraphic position in the series. The description of the occur- 

 rence of this member in ''lenses and layers" in slate by Van Hise 

 and Leith is misleading in so far as this implies that the member is 

 discontinuous within the synclinorium. The thickness of the iron 

 formation is ordinarily 50-100 ft. with a maximum of probably less 

 than 125 ft. and a minimum of only a few feet as shown in some 

 drill holes toward the center of the basin west of the Princeton and 

 Stegmiller mines. Some sections show a greater thickness than 

 125 ft., which is accounted for by folding. The formation is thinner 

 and at the same time leaner toward the west side of the synclino- 

 rium. All of the known ore bodies are on the east limb of 

 the fold. 



The iron formation is mainly banded, ferruginous chert similar 

 to the " soft ore jasper " of the other Michigan ranges. The original 

 or unaltered phase is cherty iron carbonate. North of the Swanzy 

 pit in section 18, the base of the formation, as shown by drilling, 

 seems to be mainly griinerite schist. This part of the district shows 

 evidence of greater deformation by folding and faulting than areas 

 farther south. 



The upper part of the iron-bearing member is slaty in many 

 places and the base of the overlying slate is here and there so 

 ferruginous that it is a matter of choice as to whether it should be 

 mapped as slate or iron formation. On the map these phases are 

 included in the overlying slates. 



The iron ores are both Bessemer and non-Bessemer grades, the 

 latter greatly predominating, very soft and fine textured in the 

 main and generally high in moisture. A purplish satin luster is a 

 peculiar characteristic of the Gwinn ores. There are some pits in 

 the upper part of the formation west of the Austin mine that show 

 hard jasper and hard, blue hematite. Localization of the ores is 

 largely coincident with synclinal troughs and faulted zones but is 

 not limited to these structures. An inclined position of the iron 

 formation between the overlying slate and underlying slate or 

 arkose satisfies the structural requirements for ore concentration. 



