In the attempt to correlate areas of pre-Cambrian rocks the greatest difficulties arise in bridging gaps occupied by intrusive granite. Such a gap has separated the Huronian rocks of the Vermilion iron-bearing district in Minnesota and the series of rocks on Rainy Lake (see figure 1). For several seasons the Minnesota Geological Survey has been at work in this area, and the conclusion has been reached that the mica schists of the altered Knife Lake slates, shown by the United States Geological Survey as Lower-Middle Huronian, are equivalent to the mica schists shown by Lawson on Rainy Lake as Coutchiching. Before stating such a conclusion, however, the Survey has felt it necessary to restudy certain critical areas and do detailed work at certain places on the problems involved. This can now be reported. The paper has been critically read by the members of the Survey staff and by Doctor . . .