STRATIGRAPHIC CLASSIFICATION 559 



under one name, confusion ensues and the conclusion becomes a 

 guess. 



It follows that we need to recognize, define, and name three 

 separate things: (i) lithologic individuals; (2) faunal individ- 

 uals; and (3) time intervals. 



LITHOLOGIC INDIVIDUALS. 



Definition. — Some twelve years ago there was held a confer- 

 ence of the geologists of the United States Geological Survey, 

 and at that time the basis for the Geologic Atlas of the United 

 States was laid. That foundation was planned upon the simplest 

 lines. It was proposed that the maps should exhibit the distri- 

 bution of local litholoo'ic individuals. 



Referring to the Tenth An7iual Report, Part I, in the account 

 of that conference we find the following statement (pp. 63-64): 

 "Among the clastic rocks there shall be recognized two classes 

 of divisions, viz., structural divisions and time divisions." 

 Observe that time is set off in contrast to structure, and that 

 structure is not defined by time. Then, defining structural units, 

 the report says (p. 64): "The structural divisions shall be the 

 units of cartography, and shall be A&^x^wdXo.^ formations. Their 

 discrimination shall be based upon the local sequence of rocks, 

 lines of separation being drawn at points in the stratigraphic 

 column where lithologic characters change." Proceeding to 

 emphasize that, the report further says (p. 64): "Each forma- 

 tion shall contain between its upper and lower limits either rock 

 of uniform character or rock uniformly varied in character." By 

 the latter phrase it was recognized that there might be groups of 

 lithologic individuals which singly could not be mapped because 

 too small — a difificulty which had to be met from the practical 

 side. Furthermore, the report says : "As each lithologic unit 

 is the result of conditions of deposition that were local as well 

 as temporary, it is to be assumed that each formation is limited 

 in horizontal extent ; the formation should be recognized and 

 should be called by the same name as far as it can be traced and 

 identified by means of its lithologic characters, aided by its 



