THE IRON SERIES, CONTINUED. 



109 



miles west of the Marquette mines. The rocks are less metamor- 

 phosed than in the previous two districts. The strata run east and 

 west with a northerly dip of 60 to 80, and with no subordinate 

 folds. They consist of cherty limestone at the base, followed by 

 quartz, slates, quartzite, iron ore, and ferruginous cherts, and final- 

 ly slate and schists. The strata are traversed by dikes. The ore 

 is a soft, red, somewhat hydrated hematite, with more or less man- 

 ganese, which is often considerable and is most abundant in the 

 southern mines. Hard specular is rare. Irving first showed that these 

 ore bodies had originated from the replacement of dolomitic or 



FIG. 20. Cross section of the Colby mine, Penokee- Gogebic district, Mich- 

 igan, to illustrate occurrence and origin of the ore. After C. R. 

 Van Hise, Amer. Jour. Sci., January, 1891. 



calcitic beds with iron oxide. Since then Van Hise has proved 

 them to be in the troughs formed by the intersection of northerly 

 dipping compact quartzites and southerly dipping trap dikes. . He 

 has traced the iron to a source in the layers of cherty carbonates, 

 parallel with the quartzites and above them. From this it has 

 been leached out by the percolating water and has been deposited 

 in the apices of the troughs, where it has replaced the original car- 

 bonate rocks'. Somewhat the same process is outlined for the Mar- 

 quette ores in his latest paper. Up to 1891, there were produced 

 a grand total of 8,300,000 tones of ore. 1 



1 R. D. Irving, Geol. Survey of Wisconsin, III., pp. 100-167, 1880. " Or- 

 igin of the Ferruginous Schists and Iron Ores of the Lake Superior Re- 



