DISCRIMINATION OF TIME-VALUES IN GEOLOGY 5/1 



it seen in the discussion of classification and nomenclature in the 

 Tenth An7inal Report of the United States Geological Survey, and 

 in the legends of the folio maps. For instance, take the Sewanee 

 Folio, Tennessee : The legend is as follows, viz.: 



Walden sandstone - - 1 



Lookout sandstone - - r- u -r 



L Carbonirerous. 



Bangor limestone - - i 

 Fort Payne chert - - J 



Chattanooga black shale \ Devonian. 



Rockwood formation 

 Chickamauga limestone L Silurian 



Knox dolomite - 



According to the rules in the Tenth Ammal Report, the first 

 series of names are '' structural divisions .... units of cartog- 

 raphy, and shall be designated yi?r;/ztfz'/c«.y" (p. 64). 



The second series of names are "/z;;?^ <a^/t^w/c??j- .... defined 

 primarily by paleontology and secondarily by structure, and 

 they shall be called periods'' (p. 65). 



Although everybody understands what is meant by the classi- 

 fication in the legend, the principle described in the rules is 

 wrong in that the legend on the map refers to a classification of 

 rocks : and the real fact in the case is that in the Sewanee quad- 

 rangle the Walden, Lookout, Bangor and Fort Payne formations 

 together constitute the Carboniferous system, and the map makes 

 no record of periods of time but only of formations of rocks. 

 The Devonian system of that quadrangle consists of the one 

 Chattanooga formation ; and the Rockwood, Chickamauga, and 

 Knox formations are the only representatives of the Silurian 

 system recognized on the sheet. 



The European nomenclature avoids this confusion by recog- 

 nizing a set of stratigraphic names and their categories ; with a 

 corresponding set of categories for the chronologic classifica- 

 tion — the names of the divisions being the same in both the 

 stratigraphic and chronologic scales. Instead of referring all 

 stratigraphic divisions to one category (the formation), different 



