584 E. O. IJLRICH REVISION OF THE PALEOZOIC SYSTEMS 



whose grandeur of scenery and general interest attracted many geologists. 

 Naturally we make the most of that on which we are best informed ; and 

 obviously, too, relative accessibility and ease of determining the sequence 

 of deposits are matters on which the state of our knowledge largely de- 

 pends. Thus compared the contrast in relative stability of classification 

 between the Cenozoic and Mesozoic rocks on the one hand and the Eo- 

 paleozoic formations on the other is sufficiently evident to make further 

 comment unnecessary. 



Geological ages — that is, the minor divisions of the time scale — being 

 distinguished from preceding and succeeding ages by the reversal of the 

 movement of the strandline it seems desirable, on the ground of con- 

 sistency, that the same principle should govern in deciding the limits of 

 the larger time divisions. In other words, that we should endeavor to 

 draw the boundaries between stages, epochs, and periods at horizons indi- 

 cating the greatest and most widely recognizable stratigraphic hiatus or 

 the greatest shifting and change in pattern of seas in the comprising part 

 of the column. In the case of periods it should be drawn with due regard 

 to the matter of rhythm in the progress of geological processes. (See 

 pages 398-403 and 599-607.) In the case of eras it should be at the most 

 generally recognizable horizon of this kind occurring in the midst of 

 beds that in composition and structure indicate activity of diastrophic 

 processes of the highest grade. 



Hitherto there has been no uniformity in method of drawing bound- 

 aries of time and stratigraphic divisions below the grade of eras. One 

 author drew the base of stratigraphic divisions at horizons thought to 

 represent the beginning of their typical physical and organic develop- 

 ment; another began the new formation, group, series, or system at the 

 first introdviction of species or faunas, or at the base of beds that were 

 thought to be introductory to conditions assumed on the ground of ex- 

 perience to be typical of the new age. The former method, which com- 

 monly includes that of classification by lithologic criteria and the often 

 very differently resulting method of classification by percentage or domi- 

 nance of characteristic fossils, was possibly justifiable under the old con- 

 ception of large continental seas and continuous deposition through one 

 period into the next. Under the newer conception of small conti- 

 nental seas and frequent interruption of sedimentation it was seen to 

 lead to so many inconsistencies and errors in correlation and classifica- 

 tion that it is now thought wholly indefensible. Its greatest source of 

 error lies in the fact that it entirely ignores the diastrophic factor of 

 geologic history when this factor is not clearly indicated by sharp changes 



