FORMATION THE BASIS FOR GEOLOGIC MAPPING 7^7 



mapping, and utility of the resulting maps, the lithologic unit 

 would seem to leave little to be desired. Difficulties arising 

 from diverse interpretations of the rules for its discrimina- 

 tion can be overcome with little trouble. A more serious diffi- 

 culty, however, is to be encountered in regard to the question of 

 the geographic names to be given the units. It is not sufficient 

 to say that the rules of priority will determine the name to be 

 used, even if those rules be re-formulated with special regard to 

 this particular question. The difficulty to which the writer 

 alludes can almost be said to be inherent in the definition of the 

 unit and to be avoidable only by giving a very free interpreta- 

 tion of that definition. It arises when two or more geologists, who 

 commenced work in more or less widely separated areas, have car- 

 ried areal mapping to a junction. Identical formations will then 

 bear different names on adjoining folios. In regions whose 

 geology is fairly well known, identity of nomenclature can gen- 

 erally be arranged in advance with safety. In less well-known 

 regions, however, the possibility that the same formation may 

 require treatment under different names in adjoining folios will 

 have to be accepted as a necessary evil, largely outweighed by 

 the positive advantages of adopting the lithologic unit as the 

 basis for mapping. 



Edwin C. Eckel. 

 New York State Museum, 

 Albany, N. Y. 



