irving's definition of the huronian. 107 



have had his mind on the upper Huronian. This supposition is strength- 

 ened by Irving's definition of the Huronian. He says : 



" The series of rocks here displayed may be briefly characterized as a great succes- 

 sion of quartzose laj-ers, including a subordinate quantity of graywackes [argillites 

 and slate conglomerates as here designated], a much smaller proportion of limestone 

 and chert, and numerous greenstones of eruptive origin, the latter occurring both in 

 dike and bedded form." 



Not a phrase of this is applicable to what we have understood by the lower 

 slate conglomerate, though the definitions of Murray and Logan invariably 

 embraced them, and hence Irving's opinion that describers had insisted too 

 much on the chloritic and epidotic constituents.* Hence, finally, room for 

 the conviction exists that Irving, when he identified the Animike and the 

 Huronian, had in mind only the upper Huronian. In such identification 

 we shall show that he was correct. 



DISCORDANCES IN THE LAKE SUPERIOR DIVISION. 



Logan's Views. — In the vicinity of Thunder bay, on the north shore of 

 Lake Superior, the two systems of rocks which had been so easily confounded 

 on the shore of Lake Huron present such unconformity as to render it im- 

 possible to unite them. The upper slate conglomerate presents a gentle 

 inclination, and rests on the upturned edges of the lower slate conglomerate. 

 Nevertheless, both were for a time described as different portions of the 

 Huronian, just as they had been, and still are, confounded in the "original 

 Huronian '*' (see ante, pp. 93, 97, 101). In both regions the system was con- 

 ceived as reaching from the Potsdam sandstone downward to the gneisses, 

 and that one system was Huronian. "The series of rocks occupying this 

 country," says Sir William Logan, "form the connecting link between Lakes 

 Huron and Superior to the vicinity of Shebawenahning, a distance of 120 

 miles, ^ * ^ [and] it appears to me, must be taken as belonging to one 

 formation. On the west it seems to repose on the granite; * ^ ^ on 

 the east the same supporting granite was observed by Mr. Murray. ^ ^ ^^ 

 The evidence afforded by the facts collected by Mr. Murray ^ * ^ is 

 conclusive "^ * * that successive^ormations of the lowest fossiliferous 

 group of North America'* rest nearly horizontal on these uptilted beds. 

 Thus " one formation " is included between the gneiss and the Potsdam 

 sandstone. 



At a later date Sir William Logan separated the rocks above the uncon- 

 formity from those below, and restricted the term " Huronian " to those 

 below, as first suggested by Murray in 1861. As the Huronian of Lake 

 Huron was the seat of the Bruce and Wellington mines, he designated these 

 rocks the " lower copper-bearing series," identifying with them the cuprif- 



*See note, ibid., p. 188. 



