THE 



JOURNAL OF GEOLOGY 



OCTOBER-NOVEMBER, igoi 



INDIVIDUALS OF STRATIGRAPHIC CLASSIFICATION. 



INDIVIDUALS TO BE DISTINGUISHED. 



Should geologists map the record of physical conditions or 

 the record of biological conditions — rocks or fossils? Both, 

 but with distinction. 



When the geologist enters the field to do stratigraphic work, 

 one of the first problems to confront him is where he shall 

 divide the series of rocks he is studying; and often, when read- 

 ing a paper, we are perplexed to know where these lines have 

 been drawn and what the divisions are intended to represent. 

 This discussion is an effort to arrive at a better understanding of 

 what we classify and what may be mapped. 



Since the earliest days of geologic work there has been 

 recognition of different kinds of rocks. For many years geolo- 

 gists compared the various rocks they found, and correlated 

 them from continent to continent on the basis of like physical 

 conditions represented in the similar lithologic characters of the 

 rocks, Identity of physical conditions was interpreted as indi- 

 cating the same date, but we now know that the physical charac- 

 teristics of rocks are repeated from time to time, and are diverse 

 in different provinces at the same time, and that therefore they 

 do not afford criteria of contemporaneous deposition. Further- 

 more, conditions of sedimentation are related to currents, shores, 

 and other moving features ; the zones of deposition may migrate 

 Vol. IX, No. 7. 557 



