ARCHEAN GEANITES. 265 



stone is cut by acid dikes wliicli are petrographically similar to this granite 

 and are believed to be offshoots from it. Hence the fact that the Ely green- 

 stone is older than the granite is indisputably shown. 



Although the Soudan formation does not occur near the main mass of 

 the granite, nevertheless dikes similar to the granite are found cutting 

 through this iron formation at places a number of miles distant from the 

 main massive, and if it is admitted that these dikes belong to the same 

 period of intrusion as the main mass of granite, then it is equally plain that 

 the iron formation is older than the granite. 



Relation to Loiver Huronian. — The Lower Huronian sediments and the 

 granite occur close to each other. For the most part these sediments are 

 fine slates with graywackes and very few narrow bands of conglomerate. 

 However, at a few places qonglomerates have been fomid overlying and 

 derived from the granite, and very good proof of the relations between 

 the granite and Lower Huronian sediments is thereby given. Negative 

 evidence is furthermore offei'ed by the fact that no dikes which can be 

 identified with the granite are found penetrating these Lower Huronian 

 sediments, although they occur in the greenstones that immediately underlie 

 these sediments, and are in close proximity to them. 



Belation to Keweenaivan. — The granite is itself cut by narrow dikes of 

 coarse black diabase, which are supposed to be of Keweenawan age, the 

 very youngest intrusives occurring in the district. 



GRAKITE OF SAGA:NAGA I;AKB. 



The granite of Saganaga Lake has probably one of the best-known 

 names of any geologic formation occurring in the Vermilion district of 

 Minnesota, for it has appeared repeatedly in the Minnesota reports and in 

 other publications in which its field and age relations to the adjacent rocks 

 have been discussed." 



"Winchell, A., Geol. and Nat. Hist. Survey of Minnesota, Sixteenth Ann. Rept., 1888, pp. 

 211-233 and 330-334. Grant, U. S., Geol. and Nat. Hist. Survey of Minnesota, Twentieth Ann. Eept., 

 1893, pp. 83-95; Final Rept., Vol. IV, 1899, pp. 321-323 and 467; also Am. Geologist, Vol. X, 1892, 

 p. 7. Lawson, A. C, Am. Geologist, Vol. VII, 1891, p. .324. Geological age of the Saganaga granite, 

 by H. V. Winchell: Am. Jour. Sci., 3d series, Vol. XLI, 1891, pp. 386-390. 



