﻿260 B. D. Irving — ferruginous Schists and 



smaller part having infiltrated subsequently. But that essen- 

 tially all the silica had bad a common origin became more and 

 more evident as the work progressed. 



Later, as further study developed the fact that the least 

 altered forms of the ferruginous schists contain a considerable 

 proportion of some carbonate — the amount of carbonate increas- 

 ing inversely with the amount of disturbance and alteration — 

 the idea of a possible formation of these rocks by chemical 

 deposition approximately in their present conditions gave place 

 to views which included the idea of a replacement of some 

 rock, originally dolomitic or calcitic, by siliceous and ferrugin- 

 ous substances. These latter views received considerable sup- 

 port by the discovery by Professor Van Hise that in the Peno- 

 kee region certain regularly bedded hematites could be traced 

 directly into one of the carbonate-bearing beds above referred 

 to, the thin sections showing the gradual replacement of the 

 rhombohedra of the carbonate by the red oxide of iron. That 

 the original carbonate was itself ferruginous was an idea 

 that had presented itself, but had been abandoned. But a 

 suggestion from Professor Pumpelly led to a further chemical 

 examination with regard to this point in which much new 

 material was included. The result was the establishing the 

 general ferriferous character of the original carbonate ; while 

 further quantitative analyses made in the chemical laboratory 

 of the IJ. S. Geological Survey showed that it is often even a 

 genuine siderite, though magnesian. This point once estab- 

 lished, all of our field and laboratory observations seemed to 

 fall suddenly into line and to establish the truth of an hypothe- 

 sis long since advanced, viz : that these ferruginous rocks 

 were once carbonates analogous to those of the coal measures ; 

 though the advocates of this hypothesis can hardly have 

 realized the ground it could be shown to cover, and they cer- 

 tainly did not have any conception of the nature of the altering 

 process. 



The facts that have led us to this conclusion can of course 

 be presented, in an article like the present one, in a general 

 manner only, though the details are often of importance to the 

 result. 



The ferruginous rocks of the Lake Superior region may be 

 conveniently separated into two classes, viz : those that occur in 

 unfolded formations, and those that occur in formations more or 

 less folded. Into the first of these groups fall the north-dipping 

 iron-bearing beds of the Penokee-Gogebic series on the south side 

 of the lake, in Wisconsin and Michigan, and those of the south- 

 dipping Animike series, on the northern side of the lake, in 

 Minnesota and Canada. Into the second group fall the ferru- 



