120 THE JOURNAL OF GEOLOGY. 



any one of the three formations of the Lower Huronian. As a 

 consequence the basal conglomerate may consist mainly of the 

 fragments of any one of these formations, or of all of them 

 together. Not infrequently detritus, derived from the Basement 

 Complex, is mingled with that of Lower Huronian origin. How- 

 ever, as a consequence of the resistant character of the jaspery 

 iron-bearing formation of the Lower Huronian and of mining 

 operations, the discovered contacts are most frequently between 

 the Upper Huronian and this iron-bearing formation. In the 

 basal conglomerate or recomposed rock at these points, the 

 characteristic fragments are chert, jasper, and other ferruginous 

 materials, and it is locally so rich in iron as to bear ore-bodies. 

 The uppermost horizon of the lower slate of the Upper Huronian 

 in the Penokee district is a pure, persistent layer of quartzite. 

 The central mass of the formation is a graywacke or graywacke- 

 slate, passing in places into a shale or sandstone. 



Above the lower slate is the iron-bearing member, consisting 

 of various ferruginous rocks, including cherts, jaspers, magnetite- 

 actinolite-schists, iron ores, and ferruginous carbonates. It has 

 been shown that all these varieties have been mainly derived 

 directly or indirectly by transformation from an original lean, 

 iron-bearing carbonate, which was of chemical or organic origin, 

 or a combination of both. Mingled with these non-mechanical 

 sediments is a greater or lesser quantity of mechanical detritus. 



Above the iron-bearing formation is the upper slate forma- 

 tion. This is mainly composed of shales frequently carbon- 

 aceous or graphitic, slates, graywackes and mica-schists, often 

 garnetiferous and staurolitic. The mica-schists are usually 

 toward the upper part of the formation. The stages of the 

 transformation between these crystalline rocks and plainly 

 fragmental detritus have been somewhat fully made out . 



The lower slate formation is of variable thickness, but is 

 usually less than a thousand feet. The iron-bearing formation is 

 also of very variable thickness, its maximum being perhaps about 

 the same as that of the lower slate, and from this it varies to dis- 

 appearance, the horizon being usually represented, however, by 



