728 C. K. LEITH AND R. C. ALLEN 



The general accuracy of the conclusion that the two formations are 

 of different age has been strengthened by the finding of a basal 

 conglomerate resting directly on the Negaunee formation and 

 locally cutting it out in the general vicinity of Amasa. So far as 

 this conglomerate has been traced (6 or 8 miles) it shows con- 

 clusively that the slate series on the west is unconformably above 

 the Negaunee formation on the east. Where this unconformity 

 goes to the south is not yet discovered, and hence there is still 

 doubt as to the exact location of the line of contact between the 

 Negaunee iron formation and the Upper Huronian. The extension 

 of the unconformity along its trend south or southeast in general 

 separates the formation of distinctly Negaunee type on the east 

 from the formation of distinctly Iron River-Crystal Falls type 

 associated with slates on the west. Allen, on the other hand, would 

 turn the unconformity back on itself at some place south of Amasa 

 and carry it out westward to the north of the Iron River-Crystal 

 Falls district, in order to leave the Iron River-Crystal Falls iron 

 formation with the Negaunee formation below the unconformity. 

 It seems possible that the unconformity to the south of Amasa 

 cuts out the Negaunee formation, allowing the Upper or Animikie 

 series to lap over against the underlying formation, exactly as 

 happens in places in the Marquette district. The statement that 

 the known Negaunee formation is areally connected with the known 

 Crystal Falls-Iron River iron formation is based on general similar- 

 ity of trend of the iron formation, as shown by explorations and 

 various magnetic belts. As the two have been folded together, 

 this similarity of trend is to be expected in any case. There is 

 plenty of room for the unconformity to run diagonally across this 

 general trend at almost any point. Exactly the same argument for 

 connection in the Marquette district would result in mapping 

 Negaunee iron formation with Upper Huronian iron formation, 

 whereas the two are really separated by a profound unconformity. 

 The writer is not impressed with the argument that if a threefold 

 division of the Huronian had been first made, subsequent correla- 

 tion would have been of a different sort. It is entirely true that 

 different correlations might result in starting from a twofold or 

 threefold basis of division, but the history of correlation of pre- 



