Within Minnesota from the city limits of Duluth southwestward beyond the geographical center of the state, which is lake Mille Lacs, lies a series of schistose rocks. To the southeast they disappear beneath the Cambrian sandstones, and the northwest boundary is entirely concealed by the Glacial drift. Their existence in a few localities has been known since the earliest geological explorations of the region. They were first recognized as Archean; then as Animikie—that is, Upper Huronian; and lastly, in 1894, the northernmost exposures were pronounced by Spurr to be Keewatin—that is, Lower Huronian.
The pre-Cambrian age of the series, which has rarely been doubted, is proved by the presence at two or more places of nearly horizontal sandstones in such relation to the rocks in question as to establish their subsequent age. Again, at Short Line park, near Duluth, the series under discussion is found to lie in undetermined . . .