GEOLOCilCAL EXPLORATIOXS AND LITEKATURE— ISSO. 71 



doubt as to the alteration theory with respect to the ori<)-lu of the rocks of 

 the Marquette district. He is inclined, apparently, to regard these rocks as 

 direct sediments. 



Wadsworth, M. E. Notes on the geology of tlio iron and copper distritits of 

 Lake Superior. Bull. JNIus. Com]). Zool., Vol. VII, ISSO, 1.57 pages and 6 plates. 



In this paper Dr. Wadsworth submits in great detail his views on the 

 origin of the Marquette ores. As a preface to his own observations he 

 gives a summarv of the work done in the region, as follows (pp. 2G-27): 



lu general, then, in looking over the views advocated by past observers, we find, 

 in brief, the following opinions held. 



The rocks of this district (excepting the sandstones) were all taken as Azoic by 

 Foster and Whitney, and not considered to be capable of subdivision into geological 

 periods. We must akso notice that Prof. H. D. Rogers regarded them as of Primal 

 or Potsdam age. On the other hand, we find that this formation is divided by 

 Murray, Hunt, Kimball, Winchell, Credner, Brooks, and Wright into the Huroniau 

 and Laurentiau. This division is based upon lithological characters, and on nncon- 

 formability said to exist between the two. Rivot considered the whole as Potsdam. 

 The granite is regarded as an eruptive rock by Foster and Whitney, Bigsby, and 

 Whittlesey; and as of sedimentary origin by Rivot, Kimball, Brooks, Hunt, and 

 Wright. These latter, with Credner, take it as being older than the schistose rocks 

 associated with the iron ores, and, excepting Rivot, with its accompanying gneissoid 

 rocks comi:)Osing the Laurentian formation. Foster and Whitney and S. W. Hill 

 regarded the granite as younger than, and eruptive in, the schi.sts. 



The gneisses and schists were taken by all the observers as being of sedimen- 

 tary origin, except possibly Whittlesey, whose language is as obscure as the formations 

 about which he writes. 



The metamorphism of the schists is supposed by Hubbard, Rivot, Kimball, 

 Hunt, Brooks, and Wright to be occa.sioned by chemical agencies, accompanied, as 

 part thought, by galvanism. Foster and Whitney and Bigsby considered that the 

 metamorphism was brought about by the presence of eruptive rocks, and their 

 accompanying chemical agencies. Foster and Whitney regarded the "diorites" of 

 this region as eruptive rocks, but Rivot, Kimball, Hunt, Winchell, Credner, Brooks, 

 and Wright, as sedimentary ones and interstratified with the schists. The iron ores 

 are regarded as all of sedimentary origin by Foster, Kimball, Dana, Hunt, Winchell, 

 Credner, Brooks, Newberry, and Wright, but are believed for the most part to be of 

 eruptive origin by Whitney, and by Foster and Whitney. These ores were said to be 



