THE FORMATION AS THE BASIS FOR GEOLOGIC 



MAPPING 



In a recent number of this Journal Mr. Bailey Willis, in a 

 paper on "Individuals of Stratigraphic Classification," has 

 restated and rediscussed the problem which must be solved 

 before cartographic work of any magnitude can be planned. 

 This problem involves a careful consideration of the relative 

 weights to be assigned, in any system of classification to be 

 used on geologic maps, to faunal, lithologic, and chronologic 

 (successional) characters. Mr. Willis discusses the question in 

 its various aspects, and his final decision is that the lithologic 

 unit (formation) is best adapted to the requirements of the car- 

 tographer. 



While agreeing, in ihe main, with the conclusions reached 

 by Mr. Willis, it seems desirable to call attention to certain 

 arguments, not specifically mentioned by him, which may be 

 adduced in support of those conclusions ; and, further, to exam- 

 ine the results of the application of the proposed system of 

 classification to some particular cases of interest. 



Before commencing the discussion of this question I wish to 

 acknowledge my indebtedness to Dr. F. J. H. Merrill, director 

 of the New York State Museum, who has greatly aided me, with 

 both criticism and advice, during the preparation of this paper. 



THE NECESSITY FOR UNIFORMITY 



Though the formation, defined primarily by lithologic char- 

 acters, was officially adopted in 1889 as the cartographic unit of 

 the United States Geological Survey, in practice it has not 

 entirely superseded other units of classification. Great variety 

 exists in the practice of the various state geological surveys, as 

 is indicated by their official maps ; and greater variety, as might 

 indeed be expected, in unofficial maps accompanying papers on 



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