Page:The Journal of geology (IA journalofgeology11893univ).pdf/134
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. However, 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 formation. This is mainly composed of shales frequently carbonaceous 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 disappearance, the horizon being usually represented, however, by