70 THE CRYSTAL FALLS IKOX-BEARIXG DISTRICT. 



are sixits, in whlcli is a large quantity of chert mixed with iron oxide. As 

 suoli ferrug'inous-ohert areas increase in quantity the ore grades hito the 

 terruo'inous chert and cliert wliich is found associated with it in bands and 

 lenticuhir areas. 



ORIGIN OF THE ORE DEPOSITS. 



The mode of occurrence and general characters of the ore body hav- 

 ino- lieen described, we are now prepared to determine the cause of concen- 

 tration of the iron at this particular point and the source. From the 

 description it was seen that the appearance of the body of ore was that of 

 a liedded deposit. The microscopical examination shows, however, that 

 the ore presents no evidences of clastic origin. An examination of the 

 cherts and rocks of the area which are interbedded with the ore, and 

 also a studA- of the southern contact of the ore body, shows that 

 the ore is a chemical deposit, or the result of a replacement process, 

 by wliich the original rock was largely removed, and its place taken 

 by the present ore. It has been shown (p. 62) that the siderite bands 

 pass into hematitic and limonitic chert bands. It has been seen that in 

 the southern end of the mine the lean ore merges into a mass of ore bedded 

 with chert and mixed with a rock called by the miners lime and quartz 

 rock. I interpi'et this rock to be banded siderite and chert, possibly with 

 some quartzite bands, all of which are found outcropping at the surface. 

 The siderite evidently has been changed into iron oxide and the silica 

 replaced by iron oxide, the banding of the original rock not having been 

 destroyed thereby. Irving^ considered siderite to be the source of similar 

 ore and associated chert and jasper. Van Hise^ has fully explained the 

 process of the concentration of the ores of the Penokee-Gogebic and 

 Marquette districts, and has applied the explanation to the other districts 



• Origin of tbe ferruginous schists and iron ores of the Lake Superior region, by R. D. Irving: 

 Am. Jour. Sci., 3il series, Vol. XXXII, 1886, pp. 255-272. 



- The iron ore of the Marquette district of Michigan, by C. R. Van Hise : Am. Jour. Sci., 3d series, 

 A^ol. XLIII, 1892, pp. 116-132. 



Iron ores of the Pcnokee-Gogebio series of Michigan .ind Wisconsin, by C. R. Van Hise: Am. 

 Jour. Sci., 3d series, Vol. XXXVII, 1889, pp. 32-48. 



The Penolcee iron-bearing series of Michigan and Wisconsin, by R. D. Irving an<l 0. R. Van Hise, 

 Tenth Ann. Rept. U. S. Geol. Survey, Part I, 1890, pp. 341-507. 



The Penokee-Gogebic iron-bearing series of Michigan and Wisconsin, by R. D. Irving and C. R. 

 Van Hise: Mou. U. S. Geol. Survey, A'ol. XIX, 1892, pp. 245-290. 



The Marquette iron-bearing district of Michigan, by C. R. Van Hise and W. S. Bayley, vrith a 

 chaptei- on the Republic trough, by H. L. Smyth : Mon. U. S. Geol. Survey, Vol. XXVIII, 1897, pp. 400-405. 



