Page:The Journal of geology (IA journalofgeology21894univers).pdf/462

The well known characteristic rocks of the Lower Huronian are 1) conglomerates, quartzites, quartz-schists, and mica-schists, 2) limestones, 3) various ferruginous schists, 4) basic and acid eruptives, which occur both as deep seated and as effusive rocks. The order given, with the exception of the eruptives, is the order of age from the base upward. In the Lower Huronian are placed the Lower Vermilion, Lower Marquette, Lower Felch Mountain, Lower Menominee, the cherty limestone formation of the Penokee district, and also probably the Kaministiquia series of Ontario, and the Black River Falls series of Wisconsin.

The formations of the Upper Huronian are 1) a basement slate and quartzite, frequently bearing basal conglomerates, 2) an iron-bearing formation, consisting originally of lean cherty carbonate of iron, calcium and magnesium, and 3) an upper slate. Associated with the sedimentaries in the Michigamme, Crystal Falls, and other districts, are great volcanic series, comprising greenstones, agglomerates, greenstone conglomerates, volcanic ash, and amygdaloids. Where these occur the orderly succession is destroyed. Included in the Upper Huronian are the Penokee, Mesabi, Animikie, Upper Marquette, Upper Menominee, and Upper Felch Mountain districts.

The Keweenawan consists of interstratified lavas, sandstones and conglomerates. The lavas are prevalent at the lower part of the series; interstratifications of the two occur in the middle portions; and the pure detritals exclude the volcanics in the upper portion of the series.

The Lower Huronian is largely crystalline, the Upper Huronian semi-crystalline, and the Keweenawan simply cemented. Locally along axes of intense plication, both the Lower Huronion and Upper Huronian have been transformed into completely crystalline schists. The Cambrian of Lake Superior is a horizontal sandstone, and rests unconformably upon all the preceding.

Smyth^[1] describes a contact between the lower quartzite of the Lower Huronian and the underlying granite at Republic, Michigan. Below the lowest exposures of magnetite-actinolite-schist are exposures of the lower quartzite, and below this, hanging upon the northern flank of the granite, is a conglomerate containing very numerous well rounded bowlders of granite and gneiss, identical with the rocks immediately below. It is concluded that this conglomerate from its position can not possibly belong to the Upper Huronian, and that it is a true basal conglomerate of the Lower Huronian.

Winchell, N. H.,^[2] gives the following as the general consensus of opinions