DISCUSSION OF CORRELATION 725 



tiguous districts, which lies between the well-determined succession 

 of the Gogebic on the one hand and the well-determined succession 

 of the Marquette on the other. The close folding of this area, its 

 intrusion by granite, its relatively soft character, causing low 

 rehef and few exposures, the fact that exploration and development 

 in this area are much less advanced than in the Gogebic and 

 Marquette districts, the fact that there is a general absence of dis- 

 tinctive quartzite or other formations as horizon-markers, have 

 made it difficult to carry correlations with certainty through this 

 great slate area. In general, the slates show evidence of shallow 

 water or delta deposition in their rapidly alternating bedding, 

 current bedding, ripple marks, graphitic layers, and considerable 

 thickness. In all these respects it differs from any of the slates, 

 other than Animikie, in the pre-Cambrian of Lake Superior. It is 

 substantially like the slate above the Animikie iron formation of the 

 Gogebic and Mesabi districts, like the Animikie of the Cuyuna 

 and St. Louis River districts, and like the Michigamme slate of 

 the Upper Marquette series (Animikie). Throughout the great 

 slate areas of this type in the Lake Superior region the iron forma- 

 tion has somewhat similar characteristics, being in irregular folded, 

 more or less discontinuous, lens-like bands within the slates, without 

 definitely determinable horizon. The occurrence and nature of 

 the ores are so much alike throughout these great slate areas that 

 they are naturally grouped by the explorers or mining men. 



Nearly all investigators of the geology have noted these broad 

 similarities and have correlated the slates of these districts as 

 Animikie. 



In arguing for the revision of pre-Cambrian correlation of 

 Michigan, Allen accepts the correlation of the slate series of the 

 Iron River-Crystal Falls district with the Animikie or Upper 

 Huronian of the Gogebic district, but its correlation with the 

 Upper Huronian or Animikie of the Marquette district is disputed 

 on the ground that it may be the equivalent to the Middle Huronian 

 of the Marquette district, leaving the Upper Huronian of the Mar- 

 quette district available for correlation with the newly discovered 

 Copps series. The discovery of this new series leads, naturally, to 

 a search for a more extensive occurrence in the region, but, in my 



