712 EDWIN C. ECKEL 



series of shales, with interbedded sandstones and occasional 

 thin beds or lenses of limestone. This series is well shown 

 along the banks of the Hudson River, extending almost uninter- 

 ruptedly from Fort Edward to Cornwall. Faunal differences 

 justify the paleontological division of the "group" into four 

 stages, of which the two lower carry a fauna which would corre- 

 late them with the Trenton (using that term in its paleonto- 

 logical significance), while the other two represent respectively 

 the Utica and Lorraine beds of the Mohawk valley. This divi- 

 sion into stages, however, is and must be largely theoretical, for 

 it cannot well be carried out in practice on a map. Fossils, 

 especially of species which can be regarded as of taxonomic 

 importance, are neither profusely nor regularly distributed 

 throughout the beds in question. Outcrops carrying character- 

 istic fossils are too few and far between to warrant mapping the 

 area, on a paleontological basis, on any large scale. The litho- 

 logic differences which occur in the group have no stratigraphic 

 or cartographic value, being too slight and variable to admit of 

 separate representation on a map. 



Modern geologic mapping, especially if the base of the map 

 is to be a topographic atlas sheet, in order to make adequate 

 returns for the expense involved, must be accurate within the 

 limit fixed by the scale of the map. 



The production of a geologic map is necessarily accom- 

 plished by the exercise of two functions, observation and 

 inference. Observation involves the location of outcrops and 

 lithologic boundaries with reference to fixe-d points in the con- 

 trol of the map, and is therefore purely a matter of engineering. 

 The exercise of the function of inference is necessary in order 

 to indicate the positions of boundaries concealed by superficial 

 material. Inferences in relation to such matters are dependent 

 for their accuracy on the training and judgment of the geologist; 

 his appreciation of the relations existing between structure and 

 topography, and his knowledge of the geometric effects of dip, 

 pitch, etc., in determining the position, both horizontal and ver- 

 tical, of a concealed boundary line. 



