748 JOURNAL OF GEOLOGY. 



Dakota is referred to, and, following Hall, White, and Irving, the con- 

 clusion 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 terranes includ- 

 ing 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 forma- 

 tion 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 : 



