376 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



except that the base of the Devonian is drawn at the top of the Oriskany, 

 this formation being now at the summit of the Silurian. The result after 

 these changes, when viewed in the light of present knowledge, appears 

 retrogressive rather than progressive. 



Dana's final classification was published in the 1896 edition of his 

 Manual. Several changes, all progressive, are now apparent. The Pale- 

 ozoic is divided into two parts, Eopaleozoic and Neopaleozoic. The for- 

 mer comprises two systems, the Cambrian with three subdivisions — Lower, 

 Middle and Upper, and the Lower Silurian; the latter is divided into 

 three systems — the Upper Silurian, Devonian, and Carbonic. 



The most important of the changes in the 1896 edition is the recog- 

 nition of the great break between the lower and upper Silurian and the 

 coordinate rank assigned to each of these two divisions. In so far it 

 admits the truth of Sedgewick's original contentions as opposed to those 

 of Murchison. The acceptance of a Cambrian system beneath the Lower 

 Silurian was perhaps a no less significant admission. Of minor import 

 is the return of the Oriskany to the base of the Devonian. 



Except that the name Ordovician, proposed by Lapworth in 1879, is 

 used in place of Lower Silurian, the classification of the Paleozoic adopted 

 in 1903 by the committee of leading American geologists invited by the 

 U. S. Geological Survey to revise American stratigraphic nomenclature, 

 is practically the same in its major divisions as that of Dana, 1896. So 

 far as the number and in general the components of the Paleozoic systems 

 are concerned, no important change was suggested in the eleven years 

 following the issue of Dana's latest classification. The latterly increasing 

 practice of the ~New York Survey of using their old names, Champlainic 

 instead of Ordovician, and Ontario instead of Silurian, is not taken into 

 account, since it difl^ers solely in the matter of nomenclature. 



In 1907 appeared the two volumes, entitled "Earth History," of Cham- 

 berlin and Salisbury's work on Geology. The progressive character of 

 this work is indicated by the adoption of Proterozoic, an era of sedimenta- 

 tion between the Archeozoic beneath and the Paleozoic above. Again it 

 is shown by the separation of the Carboniferous into three systems, the 

 Mississippian, the Pennsylvanian, and the Permian, and by the separation 

 of the Comanchean from the upper or true Cretaceous, thus dividing the 

 Mesozoic into four systems where formerly there were but three. In the 

 Cenozoic, on the contrary, Chamberlin and Salisbury suggest a reduc- 

 tion in the number of systems or periods, the trend of their argument 

 being that three — the Eocene, Miocene, and Pliocene — are all that are 

 justified by the application of criteria employed in the differentiation of 

 the older systems. 



