450 REVIEWS 



is a series of conglomerates, sandstones and shales. In a belt north 

 of the northern belt of Lower Keweenawan rocks, extending from these 

 rocks to the shore of Lake Superior, is the Lake Superior sandstone 

 (Cambrian). This is either flat-lying or dips slightly toward Lake 

 Superior. The junction of the sandstone with the Lower Keweenawan 

 is marked by a fault, along which the Lake Superior sandstone has 

 been depressed, in some places probably as much as several hundred- 

 feet. 



The Upper and Lower Keweenawan belts form a syncline, the axis 

 of which runs northeast and southwest through the center of the tract 

 underlain by Upper Keweenawan rocks. 



While the Keweenawan rocks of this area are the same in kind and 

 age as are the productive copper-bearing rocks of Keweenaw Point, 

 the probable unproductive character of the Douglas county rocks is 

 intimated. 



Alexander WinchelP prefaces a detailed petrographical description 

 of certain phases of the gabbroid rocks of Minnesota with a brief 

 account of the general succession in structure of formations in north- 

 eastern Minnesota. This is essentially the same as given by N. H. 

 WinchelP in Volumes IV and V of the Minnesota State Survey. The 

 correlation of this succession with the succession determined by the 

 United States Geological Survey is discussed. 



Comment. — Mr. Winchell's ideas as to succession and structure 

 determined by the United States Geological Survey are naturally 

 derived mainly from Bulletin 86 of the Survey and from the " Princi- 

 ples of Pre-Cambrian Geology" published in the Sixteenth Annual 

 Report of the Survey. However, since these reports have been issued, 

 the United States Geological Survey has done somewhat detailed field 

 work in northeastern Minnesota as a result of which the ideas of the 

 United States geologists on the succession and correlation have been 

 considerably changed. The new conclusions of the Survey are briefly 

 outlined by Van Hise in the Twenty-first Annual Report. This paper 

 should be referred to by anyone reading Mr. Winchell's discussion of 

 the correlation. 



' Mineralogical and Petrographic study of the gabbroid rocks of Minnesota, and 

 more particularly of the plagioclastites, by Alexander N. Winchell : American 

 Geologist, Volume XXVI, 1900, General part, pp. 153-162, with geological sketch 

 map of Northeastern Minnesota. 



"See summaries, Jour. Geol., Vol. IX, pp. 79-86. 



