Page:The Journal of geology (IA journalofgeology11893univers).pdf/326

Dakota is referred to, and, following Hall, White, and Irving, the conclusion is drawn that the Sioux quartzite is the oldest formation in the state. Some familiar names have disappeared from the geological section, and their places are assumed by newer but more appropriate terms, as, for example, Oneota for Lower Magnesian, St. Croix for Potsdam, while Hamilton is represented by four names applied to as many subdivisions. The term Augusta is given to the terraces including the Warsaw, Keokuk and Burlington, in place of William's term Osage which is discarded as inapplicable. The Warsaw beds of Hall are here included with the Keokuk, and the term Warsaw dropped. An error occurs in the definition of the St. Louis limestone on page 72, where it is asserted that the brecciated limestone constitutes the base of the beds in Iowa. This is the case only along the extreme margin of the beds. Seaward from the old shore line, as shown along the Des Moines in Van Buren county, the basal member consists of a brown, magnesian limestone in fairly regular, more or less undulating beds. The texture is sometimes nodular and sandy. In thickness the formation varies from five to fifteen feet or more.

The structure of the coal measures is treated in considerable detail, and emphasis is given to conclusions based largely upon Mr. Keyes' investigations in Iowa. These rocks are included in two stages, the lower or Des Moines, and the upper or Missouri formation, White's middle division being discarded. These are not considered two distinct formations in the sense that the lower was deposited prior to the laying down of the upper—the view commonly entertained—but the two were formed contemporaneously, the former as a marginal or shore formation, and the latter as its deep or open sea correlative. The view here advanced seems to be a modification of that held by Winslow. The conditions of deposition were evidently those of a slowly sinking shore, and the marginal deposits practically underlie the open sea formation though not necessarily much older; hence the terms lower and upper are retained, though emphasis is given to their general contemporaneity. The summary of Professor Calvin's researches on the Devonian and Cretaceous rocks shows a marked advance in the knowledge of these formations.

The classification of Iowa rocks, given by the different surveys, is here presented for comparison: