98 THE MARQUETTE IRON-BEAKING DISTRICT. 



this couiiectioii lie describes the Marquette Hurouiaii schists as occuiTing- in 

 intricately folded troughs. He points out that the principal dilTerence 

 between the Huronian in this region, as described by Brooks, and that on 

 the north shore ot" Lake Superior, consists in the presence in the Marquette 

 district of diorites and other greenstones, syenites, granite, gneisses, sericite- 

 schists, chlorite-schists, and talc-schists, jasper and chert-schists, augite- 

 schists, and amphibolites. Otherwise the rocks in the two areas are alike. 

 The diorites of the Marquette district are uralitic diabases. Brooks's 

 syenite is an altered diabase rich in orthoclase. The chlorite-schists fall 

 into two classes, in one of which the rocks are altered greenstones, while 

 in the other they are related to hoi'nblendic and micaceous schists. The 

 gneisses and granites rise from beneath the schists that are associated with 

 the iron-bearing rocks. Other schists which are iuterbedded with the 

 gneisses must be regarded as belonging with them. The Keweenaw^an, or 

 copper-bearing- series, is distinctly younger than the Huronian. 



On the map accompanying the report the Marquette Huronian occu- 

 pies approximately the same area as it does on Brooks's map, except in the 

 area southeast of Michigamme Lake, where the gneisses and granites are 

 colored for the Archean (pp. 166-173). 



Irving, R. D. The copper-bearing rocks of Lake Superior. Mon. U. S. Geol. 

 Survey, Vol. V, Washington, 1883, xvi and 464 pages. With maps. The Marquette 

 and Menominee regions, pages 393-409. 



The same year that saw the preliminary report on the Keweenawan 

 series saw also the full report of which the preceding paper is an abstract. 

 About the only additional idea concerning the Huronian rocks, embraced 

 in the detailed report, is imparted in one of the conclusions, which is as 

 follows : 



The Huronian sediments are metamorphic, whatever the nature of the meta- 

 morphosing process may have been — and the metamorphism has always been gi-eatest 

 where the folding has been greatest — while the Keweenawan sediments are unaltered. 

 The metamorphism and folding may have taken place before or during the period of 

 Keweenawan eruptions and depositions, or both. Our present knowledge of the 

 Huronian is too incomplete to allow of a very tirm opinion as to this point. (P. 409.) 



