Page:The Journal of geology (IA journalofgeology11893univ).pdf/322
received successive coats of other kinds of rock in after ages. The most dissimilar series of formations are known to be of the same age. What is happening to-day has happened in all ages. Nothing could be more unlike than the deposits now forming along the various ocean shores, and in different lakes and inland seas; yet they are all of one age. Even the deposits making in one and the same basin radically differ; as, for example, along the northern and southern sides of Lake Ontario; and along the eastern and western sides of Lake Champlain. It would therefore seem a useless task to seek for the Huronian rocks far from their native range.
Nason[1] fully describes the iron ores of the porphyry region of Missouri, and incidentally treats of the associated rocks. The porhyries usually show evidence of bedding, but this may be that of igneous flows. The Cambrian limestones and sandstones flank and rest unconformably upon the granites and porphyries. The iron ore of Iron Mountain and most of the other localities is in veins in the massive rock, probably of water infiltrated origin; or in a residuary mantle; or as concentrated detritus along the slopes or ravines of the porphyries. In the two latter cases the ore is derived from the veins. In some cases this concentration occurred before or during the deposition of the Cambrian sandstones and limestones, but in other cases is subsequent to the deposition of these rocks. At Pilot Knob the succession from the base upward is porphyry; conglomerate; a slaty ripple marked stratum in contact with the ore body; main ore body, nineteen to twenty-nine feet thick; highly ferruginous slate, one to three feet thick; heavy beds of conglomerate with an average thickness of one hundred feet. The pebbles of the conglomerate are mainly derived from the porphyries, but the regularly laminated slate and ore have a thin bedded structure, which is such as to lead to the conclusion that they are undoubtedly of sedimentary origin.
Bell[2] gives a general account of the Laurentian and Huronian systems, and a sketch of the geology of the country extending from Lake Huron northward to Lake Temiscaming, and from Lake Nipissing westward to the Spanish river. The Laurentian system is divided into an upper and a lower formation. The latter consists almost entirely of fundamental gneiss, while the upper Laurentian appears to consist of metamorphosed and sedimentary strata, to some extent at least.
The lower division of the Laurentian consists of red and gray gneiss, usually much bent or disturbed, and having generally a rudely foliated structure, and a solid or massive character. The feldspar is almost entirely ortho-