The middle Ordovician of the Twin City (Minneapolis–Saint Paul) region forms an isolated patch of outcrops. It is an outlier from the great area to the south and east which is covered by these formations and to which it was definitely united until very recent (probably late Tertiary) time. The characteristics of these formations, as observed in this outlier, are much the same as in the more extensive area. However, in both regions these formations differ somewhat in composition from place to place, and with this dissimilarity there is a corresponding variation in the chemical changes that have taken place since the deposition of the sediments. Recently rather extensive excavations and tunnelings have been made in the Minneapolis and Saint Paul area, which show especially the basal Decorah shale, all of the Platteville limestone, and the upper part of the St. Peter sandstone. Since these cuts are in . . .