THE STRATIGRAPHIC COLUMN 377 



The classification just published by Schuchert^^ goes much farther in 

 the way of dividing up the Paleozoic systems. First, he divides the 

 Paleozoic into two eras, restricting this name to the lower era and 

 adopting Dana's Neopaleozoic for the upper. The Paleozoic then is 

 divided into six systems, the three lower of which (Georgic, Acadic, and 

 Ozarkic or Cambric) correspond to the old Cambrian, and the three upper 

 (Canadic, Ordovicic, and Cincinnatic) to the old Ordovician or Lower 

 Silurian. The Mesozoic is as in Chamberlin and Salisbury, except that 

 the Triassic and Jurassic are merged into a single system; and the 

 Tertiary is divided into only two systems instead of three to six, as was 

 the custom previously. Although Chamberlin and Salisbury recognized 

 the high value of diastrophic criteria in stratigraphic classification, 

 Schuchert was the first to publish a scheme based wholly on one phase of 

 diastrophic activity, namely, the displacements of the strandline. The 

 principle followed by Schuchert doubtless is the most reliable so far 

 discovered, but his method of determining its operation is believed to 

 have been faulty and the result correspondingly inconsistent. Using only 

 the stages shown in his paleogeographic maps, he plotted a curvj indicat- 

 ing the alternating advance and retreat of the strandline according to 

 which he then drew his systemic boundaries. Obviously this evidence is 

 incomplete, since it includes only the stages mapped by him and disregards 

 the often more important sea withdrawals that occurred in the unmapped 

 intervals. Besides it is thought that the other diastrophic criteria were 

 not sufficiently considered. 



Comparing the last two columns of the accompanying table, it will be 

 observed that our respective arrangements agree down to the base of the 

 Silurian, except in two important features and one relatively unim- 

 portant, namely : I begin the Mesozoic farther down in the scale ; second, 

 I discard the Permic and divide its formations between the Pennsyl- 

 vanian and the Jura-Trias, and, third, prefer the term Waverlyan to his 

 restricted Mississippic. The greatest difference is found in the sub- 

 divisions of the Eopaleozoic era, Schuchert recognizing six periods, while 

 my scheme has, like the succeeding Neopaleozoic and Mesozoic, but four. 

 The reasons for these differences will be discussed in detail in Part TTT, 

 which concerns itself chiefly with the taxonomic aspects of stratigraphy. 



Diminishing imperjections of the geological record. — It will be ob- 

 served from the foregoing very brief account and the accompanying table, 

 that while the broader elements of the classification of the Mesozoic and 



^ C. Schuchert : Paleogeography of North America. Bull. Geological Society of 

 America, vol. 20, 1910, p. 606. 



XXVI— Bull. Geol. Soc. Am,, Vol. 22, 1910 



