714 EDWIN C. ECKEL 



THE DIFFICULTIES IN MAPPING ON THE LITHOLOGIC BASIS 



Difficulties will certainly be encountered in mapping any 

 large area on a lithologic basis, but these difificulties will arise 

 rather from differences m the interpretation of the rules for the 

 nomenclature of the units than from any defects in the system 

 itself. Certain cases of interest in this connection are discussed 

 below : 



Case I. — The case is that cited first by Mr. Willis (p. 563). 

 A shale passes along the strike into a limestone which retains 

 identical stratigraphic associations. " Being exactly continuous 

 stratigraphic units, they should retain the same geographic name 

 on grounds of convenience and simplicity." Considered as a 

 stratigraphic unit, the two would be discussed as the "X" 

 formation. In order to preserve all the advantages of the litho- 

 logic system of classification, however, the limestone phase 

 should be differentiated, in both discussion and mapping, from 

 the shale phase; their respective names being then the "X" 

 limestone and the " X " shale. The term " phase " seems to 

 serve a useful purpose in this connection, and the present writer, 

 therefore, proposes it for use in marking variations of sedimenta- 

 tion or of metamorphism within the stratigraphic unit. When 

 used later in this paper, it will be with this restricted and 

 definite meaning. . 



This case is illustrated excellently in New York state and 

 western New England, in this example the phases representing 

 variations in metamorphism. The Hudson shales of the central 

 part of New York state are progressively metamorphosed to the 

 eastward, becoming first slates and then mica-schists. The 

 schistose phase has been called, in Massachusetts, the " Berk- 

 shire schist;" in southeastern New York, by Merrill, the " Man- 

 hattan schist," while Dale has described the intermediate 

 phases as " Hudson slates." Throughout the entire area, though 

 differing thus in character because of variations in metamorphic 

 action, the rocks are essentially continuous stratigraphically; 

 and recognition of this fact has led to the recent proposition to 

 eliminate the names Berkshire and Manhattan, and denote the 



