264 EDWARD STEIDTMANN 



a close magmatic relationship between them and ascribes the 

 differences to differentiation. 



Analyses of fresh lavas containing o.oi to 0.03 per cent of 

 copper are cited by him as proof that the Keweenawan ores were 

 derived from the lavas. This conclusion is open to the objection 

 that a similar content of copper has been found in many rocks where 

 no ores were developed. 



Lane^ finds that the waters from the deep levels of Michigan iron 

 mines are high in sulphates and chlorides which are associated with 

 calcium rather than sodium. The carbonate radical is low and 

 decreases with depth. Their silica content is Hke that of ordinary 

 cold surface waters, and they contain almost no iron. 



A. C. Lane and A. E. Seaman^ divide the pre-Ordovician of 

 Michigan into Keewatin, Huronian, and Cambrian or Primordial. 

 The Keewatin corresponds to the Archaean; the Huronian to the 

 Algonkian; and the Cambrian or Primordial to the Keweenawan 

 and Potsdam of Van Hise and Leith. Lane believes that the 

 Potsdam and Keweenawan should not be placed in different 

 geologic periods because of their stratigraphic and structural 

 similarity and continuity, and because no unconformity can be 

 found between them. 



Lane^ recognizes the following groups of Upper Michigan 

 Keweenawan rocks from the base upward: (i) Bohemian Range 

 group: consisting mainly of basic lavas but with intrusive and 

 and effusive felsites and coarse labradorite porphorites; also 

 intrusive diabase dikes and gabbro aplites, with a total thickness of 

 about 9,500 feet; (2) the central mine group: mainly lavas of an 

 augitic ophite type with infrequent sediments; this includes the 

 Allouez conglomerate, the Calumet and Hecla conglomerate, and 

 the Kearsarge lode; the total thickness is placed at from 3,823 to 

 25,000 feet; (3) the Ashbed group: consisting of basic lavas of the 



'A. C. Lane, "Michigan Iron Mines and Their Mine Waters," Jour. Can. Min. 

 Inst., XII (1909), 114-29. 



^ Notes on the Geological Section of Michigan," part of Annual Report for 1908, 

 Michigan Geological Survey, 1909, 120 pp. 



3 A. C. Lane, "The Keweenawan Series of Michigan," Mich. Geol. and Biol. Siirv., 

 1909, 2 vols., 932 pp. 



