1 1 12 PRINCIPLES OF STRATIGRAPHY 



nal European names seemed less desirable. Thus, Taconic has 

 been used for Cambric, Champlainic for Ordovicic, Ontario for 

 Siluric, Guadaloupic for Permic. Where names proposed origin- 

 ally for series became those of systems, on the raising of tiie 

 original series to the rank of a system, they naturally differed in 

 different countries. Thus the original Subcarboniferous is known 

 as the Mississippic in America and is now regarded as a separate 

 system, while in eastern Europe it is the Donjctic and in western 

 Europe the Dinantic. The old Lower Cretacic or Infra-Cretacee ''' is 

 the Neocomic of Europe, in its broader sense, and the Comanchic 

 of America. The names of series generally differ in different coun- 

 tries, and those of stages in the different sections of the same coun- 

 try. The name in either division is derived from a typical locality 

 and the appropriate ending {ian, icn) is affixed. When the name 

 itself is not adaptable in its original form, the practice generally has 

 been to substitute the Latin form (Turonien from Touraine, Cam- 

 panien from Champagne and Carentonien from the Charent). 

 Sometimes the name is derived from the original name of the 

 locality, as Cambrian from Cambria, the old Roman name for 

 North Wales, and Cenomanien from Coenomanum, the old Latin 

 name of the town of Mans in the Department of Sarthe and Roth- 

 omagien from Rothomagus, the Roman name of Rouen. 



Mapping. 



The question is often asked : Should geologic maps express 

 primarily formations or geologic horizons? In other words, should 

 the mapping be based on lithic formations or on time units? The 

 decision generally has been in favor of the mapping of lithic units 

 or formations. Generally the units have been small enough to 

 allow a grouping into systems, and these have then been referred 

 to their proper time period. This method is apparently the most 

 satisfactory, since all mappable features, such as the outcrops 

 themselves, as well as the topography of the region, are the direct 

 consequence of the lithic formations, and have no regard whatever 

 to time relations. The present outcrops show only the present ex- 

 tent of the formations, and give no clue to the former extent of the 

 strata deposited during a given time interval, except in so far as 

 the lithic character of the formation indicates this. 



Mapping on Formational Basis. The United States Geological 

 Survey has adopted the formation as its cartographic unit, mapping 



* In recent classifications this term is discarded. See Haug Traite, p. 1170. 



