DISCUSSION OF CORRELATION 703 



formity at its base and the bearing on the general problem of the 

 enormous scale of the bathoKthic granitic intrusions of late Animikie 

 time. 



DISCUSSION OF CORRELATION 



C. K. LEITH AND R. C. ALLEN 



INTRODUCTORY STATEMENT 



The non-productive eastern end of the Gogebic Range is folded, 

 faulted, and associated both with intrusive and extrusive rocks. 

 In the lack of sufficient exposures and exploration scarcely more 

 than lithologic mapping was possible when the range was worked 

 over by Irving and Van Hise."^ Structural mapping was not 

 attempted. In recent years, exploration of this area by drilling 

 has connected some of the isolated outcrops of iron-formation 

 bands and has made it possible to map the area structurally. 

 This has been done effectively by Messrs. Allen and Barrett, who 

 outline their results in the preceding paper. While there are still 

 some parts of the area to which the structure and areal connections 

 are obscure, certain features of exceptional interest from the stand- 

 point of general Lake Superior geology have already been worked 

 out. These are: (i) discovery of the fact that there is an intrusive 

 granite with important metamorphic effects in the supposed 

 eastward continuation of the Gogebic iron formation (Animikie); 

 and (2) the existence of a ferruginous graywacke formation with 

 jasper and chert bands, called the Copps formation, separated 

 from the inferior Animikie series by a great structural uncon- 

 formity and unconformably below the Keweenawan. 



These facts raise interesting questions as to revision of the 

 pre-Cambrian correlation of Michigan, which touch more or less 

 also the correlations of the Lake Superior region as a whole. Leith 

 thinks it possible that the Copps series may be of only local sig- 

 nificance and require no general change, but he agrees with Allen 

 that Michigan correlation should be carefully tested to ascertain 

 whether the known facts can be better expressed by a different 



' Monograph ig, U.S. Geol. Survey, 1892. 



