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ity may possibly be due in part to highlands of the Basement Complex, which were not covered by the Lower Huronian sea until the period was well advanced. Of the extent of the series at the end of the erosion preceding Upper Huronian deposition, little has been determined, since later erosions have undoubtedly removed large areas of the series, and therefore its present distribution is not a safe guide to its distribution at the close of the erosion interval referred to.

The Upper Huronian.—At the close of the long period of erosion which followed the Lower Huronian deposition, the water once more advanced upon the Lake Superior region, and the Upper Huronian series was deposited.

Lithologically this series consists of conglomerates, quartzites, graywackes, graywacke-slates, shales, mica-schists, ferruginous slates, cherts, jaspers, ferruginous schists and igneous rocks, including both lava flows and volcanic fragmentals, as well as basic and acid intrusives. The series, as a whole, is very much less crystalline than the Lower Huronian, although locally the shales and graywackes have been transformed into mica-schists, and even into gneisses.

The Upper Huronian immediately about Lake Superior is divisible into three formations, a lower slate, an iron-bearing formation, and an upper slate, the basis of separation being that of mechanical and non-mechanical detritus. The inferior formation is mainly a quartzose slate or shale, but locally it passes into a quartzite, while the basal horizon is frequently a conglomerate. The nature of this conglomerate varies greatly, depending upon the character of the underlying formation, which, in some areas, is the Basement Complex, and in others the Lower Huronian. In the first case the slates may rest upon the gneissoid granite, upon the schists, or upon the junction of the two. The basal conglomerate corresponds in its character, being a recomposed granite or granite-conglomerate, a recomposed schist or schist conglomerate, or finally a combination of the two.

When the lowest member of the Upper Huronian rests upon the Lower Huronian series, the underlying formation may be