FORMATION THE BASIS FOR GEOLOGIC MAPPING 709 



areal geology, even though the authors be connected with sur- 

 veys whose official practice is uniform. 



In considering the representation on a map of the geologi- 

 cal features of a small, isolated area, the question of the taxo- 

 nomic unit to be employed is usually of minor importance ; 

 though even in this case, as shown later, there would seem to be 

 good reasons for the adoption of the lithologic individual as 

 this unit. The interest attaching to this question increases 

 directly with the size of the area to be mapped ; and the prob- 

 lem becomes of paramount importance when the independent 

 work of several geologists is to be combined, as in the compi- 

 lation of a geologic map of an entire state or other large area. 



In view of these facts, and of the slow but steady spread of 

 the geologic-folio work carried on by the United States Geologi- 

 cal Survey, it would seem necessary that the various local sur- 

 veys should adopt the same general system of classification for 

 map work. In the present state of our knowledge, detailed 

 chronologic classification, the unit for which would be the 

 epoch' of earth-history, is impossible; and the problem of unifi- 

 cation is therefore resolved into the choice between two alterna- 

 tive systems, based respectively upon biologic and lithologic 

 characters. In the opinion of the writer a geologic survey, 

 whether state or national, can best accomplish the work for 

 which it is intended by adopting as its cartographic and taxo- 

 nomic unit the formation, defined as a lithologic individual. 



THE RELATIVE SCIENTIFIC VALUES OF BIOLOGIC AND 

 LITHOLOGIC UNITS 



For the purposes of the present discussion it is necessary to 

 point out that our knowledge of the two histories [i. e., of sedi- 

 mentation and of life), so far as that knowledge can be expressed 

 on maps, is decidedly different in grade. The present phase of 

 earth-history may be examined for a determination of the truth 

 of this statement. We can map without difficulty the topo- 

 graphic features of the earth ; and the possible accuracy and 



' The word epoch is here used in an entirely general sense. 



