Page:The Journal of geology (IA journalofgeology11893univers).pdf/108

the paper gives any evidence, all of the igneous activity and dynamic metamorphism are pre-Potsdam.

^[1]Wadsworth gives a sketch of the iron, gold and copper districts of Michigan. The Azoic or Archean rocks are divided from the base upward into Cascade, Republic and Holyoke formations. These divisions are placed in order as equivalent to the fundamental complex, lower Marquette series and upper Marquette series of Van Hise. They are unconformable and represent three different geological ages. The Keweenawan is divided into two divisions, both of which are placed in the Cambrian; the Lower Keweenawan, 25,000 ft. of interbedded conglomerates and lava flows, with some intrusives; Upper Keweenawan, 12,000 feet of sandstones and shales, not separable from the Potsdam or Eastern sandstone.

The Azoic or Archean system consists of rocks formed (1) by mechanical means, (2) by eruptive agencies, (3) by chemical action.

The Cascade, or oldest formation of sedimentary and eruptive rocks, consists, commencing with the oldest, of gneissoid granites or gneiss, basic eruptives and schists, jaspilites and associated iron ores, and granites, although the above arrangement may be considered no more than a hypothesis, and it is probable that the jaspilites and iron ores will be found to belong to the Republic formation. It is also probable that the Cascade formation itself will prove to be composed of two or more distinct geological formations, as shown by the fact that the chief rock of the Huron Mountains appears to be a gneissoid granite, rather than a true sedimentary gneiss. True sedimentary gneisses are found in the Huron Bay and Cascade districts. In the former area they contain fragments that closely resemble the gneissoid granites, and thus they appear to be formed from the debris of those rocks. If, however, the gneissoid granites are metamorphosed eruptive rocks, and not true gneisses (which are restricted to metamorphosed sedimentary rocks), this fact proves only that the gneisses are younger in order of time, but not of necessity of younger geological age. Similar statements apply to the breaks between the Cascade and Republic formations, and the break between the Republic and Holyoke formations. In the Huron Bay, Menominee and other districts the Cascade formation holds intrusive granites. Amphibole-schists are also found intrusive in the gneisses in the Cascade area. In the Marquette area the amphibole schists are cut by felsite or quartz-porphyry.

Much of the granite and felsite appear to have been erupted during the time of the Cascade formation, and perhaps even later. On the Cascade range hornblende-gneiss cuts the country rock. These dykes are cut by