THE DISCRIMINATION OF TIME-VALUES IN 

 GEOLOGY. 



The imperfection of the present systems of classification and 

 correlation of sedimentary rocks concerns more directly the 

 interpretation of the facts than the facts themselves. 



The work of the geologists of the United States in mapping 

 and recording the stratigraphic sequence of formations was never 

 more exact and comprehensive. The paleontologist was never 

 more particular in his records of the faunal contents of each 

 formation and fossiliferous zone, and his comparisons were never 

 more full and precise. But the extension of knowledge over 

 vast territory has brought to light hundreds and thousands of 

 outcrops of the same formations, showing a diversity of faunal 

 composition which cannot be translated entirely into difference 

 in geologic age. 



So long as surveys were confined to local areas separated by 

 spaces across which the continuity of formations could not be 

 traced, it was practicable to use a system of nomenclature and 

 classification in which lithologic formations and their strati- 

 graphic succession were chiefly considered. When, however, 

 the intervals between local areas were filled up and it was neces- 

 sary to correlate geological sections in which the formational 

 divisions are in part or wholly dissimilar, the duality of the 

 lithologic and biologic facts become apparent. These two 

 sets of facts are entirely different in nature and in origin, and 

 for their scientific discrimination duality of nomenclature is 

 essential. 



The confusion of these two kinds of evidence was natural, 

 and has been perpetuated by the common practice of adopting 

 the lithologic formation as the unit of classification, making the 

 time divisions to apply strictly to the formations instead of to 

 the faunas and floras, by which alone the chronologic epochs in 

 which they were formed can be discriminated. This confusion 



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