206 T. B. Brooks — Youngest Huronian Rocks. 



by Abesser, Jani and Marcker in their paper on the 



f phospho 

 nd all th< 



paper 



I 



! results show that i 

 ration of silicic acid neces 

 the estimation of pbosphoricacid by the raolybdic method. 

 Prof. Kolbe'a LabOTatory, Leipzig, Dea 17, ISYS. 



Art. XXVIIL— (9ri the youngest Huronian Rocks smith of Lake 

 Superior and the age of the Copper -bearing Series ; by T. B. 

 Brooks. 



In the sammer of 1874, Chas. E. Wright and myself, while 

 exploring the country west and south of the Menominee River 

 about ninety miles from its mouth, under the auspices of the 

 Wisconsin Geological Survey, observed a lai^e granitic area, 

 the north edge of which was bounded by dark-colored horn- 

 blendic and micaceous schists of Huronian age, which I have 

 since concluded are the equivalents of the youngest member of 

 that series yet observed in the Marquette Iron B^gion.* The 

 prevailing form was a medium to c 

 with rectangular crystalline facets 

 passed through gneissoid granite t( 

 once hornblendic, the schistose struc 

 formed with the underlying schists. 



The lithological character of this wide granitic belt bore so 

 much general resemblance to the Laurentian rocks, which are 

 extensively developed on the waters of the Sturgeon Eiver in 

 Michigan, 10 to 20 miles to the northeast, that we were dis- 

 posed at the time to believe that some phenomena of folding or 

 faulting had brought rocks belonging to that system to the sur- 

 face in an unexpected quarter. Professor Pumpelly and my- 

 self, several years previously had observed, farther to the north 

 and west, similar granitic rocks crossing the Michigamme and 

 Paint Rivers (branches of the Menominee), presenting similar 

 puzzling relations with beds known to be Huronian. This for- 

 mation is noticed in my Michigan Report, vol. i, p. 175, and 

 the probability of its being Huronian, and younger as well as 

 lithologically different from any rocks then known to be of that 

 period, is pointed out.:}: 



130, Michigan Geological Report. 



t A few smaU granite dykes we 

 along the granite border. 



% It is not improbable that son 

 Lake in the Marquette Region, may belong to t] 



