﻿258 R. D. Irving — Ferruginous Schists and 



present indurated and silicified condition, in some cases at least, 

 before the production of the next overlying layer of the series. 

 It was soon seen, too, that a further proof of this early indura- 

 tion — although it does not necessarily set the induration so far 

 back — is afforded by the extraordinary crumpling, shattering 

 and brecciation which some of the jaspery and cherty ferrugin- 

 ous schists show where the formations containing them are 

 folded. The adjacent rock beds, while often folded closely, 

 show no signs of having offered so great resistance to the fold- 

 ing process. Relatively great induration antecedent to the fold- 

 ing process being thus demonstrated, the view ordinarily held 

 by those who maintain for these materials some sort of a sedi- 

 mentary origin, viz : that they have reached their present condi- 

 tions by a regional metamorphism accompanying the folding of 

 the series, seemed to have no standing ground. Many other 

 considerations, moreover, led to the same conclusion. Among 

 these I may mention the fact that these same ferruginous ma- 

 terials occur in formations in the Lake Superior country which 

 are quite without folding, and which our later researches have 

 demonstrated to be free from anything like a genuine meta- 

 morphism, using that term in the sense of a more or less thor- 

 ough molecular rearrangement and recrystallization in situ. 

 Indeed, among the folded iron-bearing rocks themselves we 

 find but little if any indication that a genuine recrystallization 

 has taken place. The quartzites, greywackes, etc., of these 

 formations retain to the full their original fragmental structure, 

 and, even in the case of the mica-schists, the changes that have 

 taken place are often demonstrably of a metasomatic nature only. 

 It seems difficult, therefore, to understand how a metamorphos- 

 ing process, which could leave the fragmental texture in the 

 adjoining rocks still recognizable, could yet have so greatly 

 altered the iron-bearing layers. Again, the silica, which fre- 

 quently forms so prominent a part of the ferruginous schists, 

 and which is at times jaspery, but is far more frequently cherty 

 or even chalcedonic — a point hitherto quite unrecognized in 

 publications on this subject — presents every evidence, both in 

 the thin section and in the field, of a chemical origin. In the 

 thin section — in that it shows ordinarily no trace of a fragmental 

 texture, even when relatively coarse-grained, and in that it 

 approaches more commonly to the peculiar chalcedonic or even 

 amorphous forms known to occur only with silica deposited 

 directly from solution ; besides which it traverses and follows 

 the banding indifferently and in such a manner as to place its 

 secondary nature beyond all doubt. In the field — in that the 

 jaspery or chalcedonic silica, while in the main closely inter- 

 banded with the more ferruginous portions of the rock, is seen 

 also to intersect the bands, or even to appear in the shape of a 



