ORE DEPOSITS Ilf MANSFIELD SLATE. 71 



in the Lake Superior region. I shall not do more, therefore, than to add 

 that the investigations in this area have shown the probable correctness of 

 this explanation. 



It is very interesting from an historical standpoiiit to note that as far 

 back as 1868 Credner had made the suggestion, with reference especially to 

 the Marquette district, that the ores were derived from an original iron 

 carbonate. The following quotation will show his idea of the processes of 

 development of the ore:^ 



Spbaerosiderit warde aiis kolilensaurereichen Gewiisseii abgesetzt, durcli eine 

 theilweise Oxydation desselbeu entstaud Magneteisensteiu, durch weitere Aufnabme 

 vou Sauerstoff das Gemenge von Magneteiseustein imd Kotheisenstein und eiidlich 

 reiner Eotheisenstein ; aus diesem sporadisch durch Zutritt von Wasser Brauneisen- 



steiu. 



Credner's suggestion seems to have been lightly considered by other 

 workers in that area. In 1886 Irving^ suggested the theory of replacement 

 of an original feiTuginous carbonate to explain the Penokee-Gogebic iron 

 ores. This theory has since then been elaboi-ated by Van Hise, and shown 

 to have a wider application to the other Lake Superior ore districts. He 

 has also traced the iron to its source in the rocks removed by denudation, 

 and shows why it occurs in the positions in which the ore bodies are at 

 present found to occur. Moreover, Van Hise has also explained the process 

 of development in detail, and, what is perhaps far more important, the 

 reason certain ores develop and not others. In its essentials, however, the 

 process is the same as that suggested by Credner in the lines quoted above, 

 though in them no suggestion of the replacement to which is due the 

 em-ichment of the ore bodies is made. 



Much of the iron of the Mansfield ore is presumed to have resulted 

 directly from the alteration of the ferruginous carbonate in place, but a large 

 amount was brought in from above by infiltrating waters. The ferruginous 

 matter, which was taken into solution during the denudation of the area, has 

 been carried down by percolating waters and deposited at places favorable 

 for its accumulation. The beds are now on edge, offering the most favorable 

 condition to percolation. The conclusion is obvious that these deposits 



'Die vorsilnrisolien Gebilde cler "Oberen Halbiusel von Michigan" in Nord-Amerika, by H. 

 Credner: Zeitschr. deutsch. geol. Gesell., Vol. XXI, 1869, p. 547. 



- On the origin of the ferruginous schists and iron ores of the Lake Superior region, by E. D. 

 Irving: Am. Jour. Sci., Vol. XXXII, 1886, p. 263. 



