STRATIGRAPHIC CLASSIFICATION 5^7 



nomenclature, in which he clearly stated the distinction which 

 the present writer would now emphasize between a lithologic 

 individual and a faunal unit. 



Following the language in which a formation is defined in 

 the Tenth Amiual Report, a faunal unit may be defined as fol- 

 lows : 



LITHOLOGIC INDIVIDUAL. FAUNAL UNIT. 



" The structural divisions shall be The faunal units shall be units of 



units of cartography, and shall be correlation, and shall be designated 

 designated formations. Their dis- stages.' Their discrimination shall 

 crimination shall be based upon the be based upon their fossil content, 

 local sequence of rocks, lines of lines of separation being determined 

 separation being drawn at those at those points in the stratigraphic 

 points in the stratigraphic column column where faunas change, 

 where lithologic characters change." 



Pursuing the description of a faunal unit, or stage, we may 

 say further : As a faunal unit is characterized by the life which 

 it contains, and as organisms are migratory, it is not to be 

 assumed that a stage is limited horizontally. It may be recog- 

 nized in diverse provinces or continents, and therefore the name 

 which a faunal unit receives should not be a local name, but 

 should be applied wherever that unit is recognized. 



Distinctions between formation and stage. — A formation is a set 

 of strata characterized by and limited to uniform constitution, 

 A stagfe is a set of strata containing and limited to a certain 

 fauna. 



Lithologic constitution results from conditions which are 

 local and temporary, and which, though migratory, are rarely 

 more than provincial in extent. A fauna, though evolved in 

 adaptation to local conditions, may be capable of world-wide 

 migration. A formation, therefore, is geographically limited ; a 

 fauna is not, necessarily. 



Physical conditions which determine rock constitution are 

 recurrent and repeat the deposition of similar sediments. But 

 organisms, once extinct, do not reappear. Accordingly, a 



' The term stage is here used provisionally, to afford a word for the purposes of 

 this discussion only. 



