﻿270 B. D. Irving — Ferruginous Schists and 



suggest that the two former minerals have resulted from it by 

 virtue of a lower degree of oxidation and the combination 

 with silica of some of the iron, along with the accompany- 

 ing magnesia of the carbonate, to form actinolite. These 

 actinolitic magnetite-slates, it should be said, also often 

 contain carbon, and the idea is suggested at least that this 

 carbon has had to do with preventing a more complete oxida- 

 tion of the iron. The silicification process has, however, in 

 large measure, removed the iron of the carbonate completely 

 and such iron has passed off in solution, to be concentrated in 

 other portions of the mass in the shape of seams or bodies of 

 the sesquioxide. 



The facts summarized in the foregoing paragraphs appear to 

 lead of necessity to the following conclusions : 



(1.) The original form of the beds of the iron-bearing hori- 

 zons of the Lake Superior region was that of a series of thinly 

 bedded carbonates interstratified with carbonaceous shaly layers, 

 which were also often impregnated by the same carbonate. 

 This carbonate was generally more or less highly ferriferous, 

 though probably there were intermediate forms between it and 

 dolomite. With these unaltered layers there seems to have 

 been some little magnetite, in the shape of a fine dust mingled 

 with the carbonaceous impurities, but most of the magnetite 

 now found is taken to be secondary. Such a series of layers 

 finds parallels in the carbonates of the coal measures, — which 

 present us, in some cases, with bands of an iron carbonate in- 

 terstratified with carbonaceous seams — and in such bedded 

 carbonates as those on the east side of the Hudson River, about 

 six miles below Hudson, N. Y. With these latter ores also 

 there is much carbonaceous matter in bands. The so-called 

 black band ores of the coal measures are also at times very 

 close in appearance to the black slaty ferruginous layers of the 

 Animike series.* 



(2.) By a process of silicification these carbonate-bearing 

 layers were transformed into the various kinds of ferruginous 

 rocks now met with in this region. The silicification varied in 

 desree from those cases where a few thin seams of silica trav- 

 erse the otherwise unchanged rock, or follow its lamination, to 

 those in which there was a complete substitution, the iron be- 

 ing wholly leached out, in which case the lighter-colored cherty 

 forms were produced. 



(3.) The iron thus removed from the rock at the time of 

 silicification passed into solution in the percolating waters, to 



* Moreover, these similarities are not macroscopic only, but extend also to the 

 thin sections, a large number of which have been made for comparison from coal- 

 measure ores, and from the Hudson ores above mentioned. 



