498 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



special request. Although professing to indicate the microscopic work 

 which had been previously done, he ignores completely that given in the 

 pages before mentioned, which anticipated him in almost every point. 

 (Am. Jour. Sci., 1883, (3) XXVL, pp. 27-32, 155.) 



In 18GG Prof. James Hall referred some gneiss and granite on the 

 Redwood Kiver to the Laurentian, and some quartzites in the valley of 

 the Minnesota River to the Huronian. This was done solely on litho- 

 logical grounds, while the two series of rocks were not seen together. 

 (Trans. Am. Phil. Soc, 1869, (2) XIII., pp. 329-340.) 



It would seem that Dr. F. V. Hayden held that the quartzite was 

 most probably supra-Carboniferous, Triassic, or possibly Cretaceous. 

 This was on account of fossils which were found in some quartzite in 

 the adjacent region. (Am. Jour. Sci., 1867, (2) XLITL, pp. 15-22). Part 

 of Professor Hall's views were regarded as untenable by Prof. N. H. 

 Winchell. (Bull. Minn. Acad. Sci., 1874, pp. 100, 101.) 



In 1872 Prof. N. H. Winchell divided the "granitic and metamor- 

 phic rocks " extending across Miimesota from the northeast to the 

 southwest into Laurentian and Huronian ; on what grounds does not 

 appear. (Report on the Geological Survey of Minnesota, 1872, pp. 64- 

 67.) 



In 1880, in the Report of the Geological Survey for 1879 (p. 26), 

 Professor Winchell remarks : — 



" We hence see the Potsdam in its extension to Duluth involved with these 

 igneous rocks, in upheaval and metamorphism, and cannot resist the conviction 

 that the whole series known as the Upper Copper Bearing Rocks, or as the 

 Keeweenian, or as the Quebec Group, on different authorities, was correctly 

 assigned to the Potsdam at first by Messrs. Foster, Whitney and Hall in 1849, 

 and suljsequently by D. D. Owen." 



The evidence on which he relies in making this statement is the fact 

 that the copper-bearing rocks at Duluth are seen to be continuous with, 

 and to form part and parcel of, the Potsdam sandstone as it extends 

 from that point into Wisconsin. This view of Professor Winchell's 

 harmonizes with that which has already been shown by us to exist in 

 Michigan. The Minnesota geologists have not furnished any evidence, 

 other than that based on lithological resemblances, to uphold the divis- 

 ion of the Azoic system into the two series Huronian and Laurentian. 

 Moreover, it seems to be the fact that in this State, as well as in Michi- 

 gan, the supposed Keweenawan is nothing more than Potsdam. 



