294 E. 0. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



in the construction of paleogeographic maps. At the same time their 

 study affords apparently the only reliable grounds on which a natural yet 

 practical classification of time and stratigraphic units may be based. 



GHAMBERLIN AND SALISBURY'S CLASSIFICATION 



As my views concerning a natural l)asis of geologic time division are 

 essentially those of Chamberlin and Salisbury,* it follows that 1 regard 

 tlieir classification of the American stratigraphic cohunn as a change in 

 tlie riglit direction and the best yet published. The chief merit of their 

 classification lies in the endeavor to make the systems approximately co- 

 ordinate in time value. In the Paleozoic part their scheme is somewhat 

 inconsistent in the matter of drawing the systemic boundaries and faulty 

 because of miscorrelations and lack of information. For example, in 

 the case of the Devonian, also the Pennsylvanian, they draw the lower 

 boundary at the beginning of a submergence following a period of broad 

 emergence. The base of the Mississippian, on the contrary, is drawn 

 above the Chattanooga shale ; hence considerably later than the beginning 

 of the great submergence that followed the final Chemung emergence of 

 the Devonian. Eegarding the respective bases of the Silurian and Ordo- 

 vician, incorrect correlations of formations are responsible for incon- 

 sistencies in their delineation. In the sections of New York and the 

 Appalachian Valley the base of the Silurian is drawn at the beginning of 

 the epoch of greatest retreat of the Ordovician sea. In the Ohio and 

 Mississippi valleys and in the far west it is drawn at some horizon suc- 

 ceeding the deposits of the first great advance of the Silurian seas. The 

 base of the Ordovician varies for other reasons, but chiefly because it was 

 thought to occur at some undeterminable horizon within a vast thickness 

 of limestone. 



It is only in the past 10 or 12 years that, as I believe, the true relations 

 of the Cambrian to the Ordovician have begun to be understood. Most 

 of the information gathered on the subject during this time is as yet un- 

 published, but the gist of it will be presented on this occasion. The 

 study of the Neopaleozoic rocks has shown that their diastrophic his- 

 tory also has not been adequately considered, and that the classifica- 

 tion of their stratigraphic units is greatly in need of revision. Unfor- 

 tunately, the facts on which the proposed innovations concerning these 

 later deposits are based can not be discussed as fully now as it is hoped 

 they may be in the future. The facts are in hand, but space is lacking.*^ 



* Geology, vol. ill, 1906, pp. 191-196. 



^ Since reading these "introductory chapters" at the Baltimore meeting of the Society, 

 Prof. Charles Schuchert's great work on Paleogeography of North America, presented at 

 the same session, has been published. Although based on the same general considera- 



