534 THE JOURNAL OF GEOLOGY. 



the paper gives any evidence, all of the igneous activity and dynamic 

 metamorphism are pre-Potsdam. 



r Wadsworth gives a sketch of the iron, gold and copper districts of Michi- 

 gan. The Azoic or Archean rocks are divided from the base upward into Cas- 

 cade, 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, con- 

 sists, 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 neces- 

 sity 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 Mar- 

 quette 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 



1 A Sketch of the Geology of the Iron, Gold and Copper Districts of Michigan, 

 M. E. Wadsworth, Rep. State Board Geol. Sur., Michigan, 1S91-2; pp. 75-174; 

 Lansing, 1893. Also, see Annual Reports 1888-1892, ibid., pp. 38-73. 



