130 C. R. Van Rise — Iron Ores of Michigan. 



tions, the ores occupy troughs or the sides of synclinals. That 

 the ore is a concentrate from an original impure iron-bearing 

 carbonate is easily shown. Wholly unaltered carbonate, often 

 accompanied by organic matter, is abundant, and all stages of 

 the change into the soft hematites and limonites of the Upper 

 Marquette series are seen. 



Ores of other districts. — In the Menominee district the ores, 

 as in the Marquette district, occur in two formations, one of 

 which belongs to the Lower Menominee, correlated with the 

 Lower Marquette series, while the other belongs to the Upper 

 Menominee, correlated with the Upper Marquette series. The 

 Upper Menominee is more important as an ore-producer than 

 the Upper Marquette. A detailed study has been made of 

 only a small number of the mines, but a general study has 

 covered nearly the entire district. So far as work has gone, 

 while there will be some modifications of detail, the principles 

 have been found thus far to hold that the ores are secondary 

 concentrations upon impervious formations, and are particu- 

 larly likely to be of large size when these are folded or two 

 combine to form pitching troughs. Also, as in the Marquette 

 district, the basement impervious formations are often igneous 

 and not infrequently are surface volcanics. At other times, 

 and especially in the Upper Menominee series, the impervious 

 stratum is a detrital slate. As typical instances of the Menom- 

 inee mines may be mentioned the following : At the Armenia 

 the ore-body is at the bottom and upon the sides of a synclinal 

 trough, pitching at an angle of about 45°. Below the ore is 

 an impervious black slate. At the Mansfield mine the ore- 

 body nearly vertical laterally but pitching longitudinally, has 

 impervious slates upon each side, one of which is clearly a 

 volcanic rock. The ore of the Hemlock mine rests upon an 

 impervious stratum consisting of surface volcanic material. 



The ores of the Vermilion Lake district have been studied 

 by us only in a general way, but so far as our investigation has 

 gone all the facts bear toward the conclusion that the princi- 

 ples here hold which are applicable to the other iron-bearing 

 districts of the Lake Superior region. It is true that Prof. 

 !N". H. Winchell and H. Y. Winchell* have proposed a chemical 

 theory for the origin of the rocks of the iron-bearing formation 

 which does not derive it from a lean iron-bearing carbonate ; 

 but even if this theory be accepted (and I do not hold it), it is 

 applicable to the ore-bearing formation rather than to the ore- 

 deuosits. The descriptions and figures of the mines published 

 by the late Prof. Alex. Winchell and by Prof. K H. Winchell, 

 seem to me to be wholly in harmony with our own observa- 

 tions, and to indicate that the Vermilion Lake ores are sec- 



* The Iron Ores of Minnesota, 1ST. H. and H. V. Winchell: Bulletin No. 6, Geol. 

 and Nat. Hist. Survey of Minn., Appendix A, pp. 391-399. 



