REPRESENTATION OF GEOLOGICAL FORMATIONS 693 



lines and planes of sedimentation, or natural sequence, to use a 

 more comprehensive term. It gives a foundation for deter- 

 mining geologic structures and deciphering geologic events. 

 This foundation is the same as that governing sedimentation and 

 the sequential arrangement of rock-masses. The scheme is, 

 therefore, genetic. It is of secondary importance to consider 

 the composition and contents of the various lens-shaped figures. 

 In some cases most value must be placed upon the lithologic 

 character of a terrane. In other instances, the contained fossils, 

 or minerals, are the determining factors. Under certain con- 

 ditions still different features must be taken into consideration. 

 The cardinal fact to be always recognized is that the cartographic 

 unit, the geologic formation, is essentially an abstract concep- 

 tion. It may also be a lithologic, or faunal, or mineralogic, or 

 physiographic, or some other kind of unit capable of being 

 represented on maps. 



As a matter of fact the lithologic features, the faunal char- 

 acters, the mineralogic contents, as well as many other criteria 

 of discrimination, are so grouped genetically to the depositional 

 units which it is desired to represent on the map, that if the 

 decipherable record of each were perfect, a cartographic repre- 

 sentation of the one set of facts would in a general way indicate 

 the probable outline of each of the other sets. But the fact that 

 the records of all of these groups of data for the determination 

 of the geologic formations are at best comparatively so meager, 

 makes it incumbent upon the geologist to delimit his carto- 

 graphic units at first according to the most obvious features 

 presented in the several areas covered. 



It so happens that in the field the most obvious and most 

 useful single feature in recognizing and tracing a geological for- 

 mation is the lithologic. Checked by other criteria, then, the 

 lithologic unit corresponds very closely to the ideal cartographic 

 unit established. For all practical purposes for which the 

 geologic map is constructed lithologic individuals are amply 

 sufficient and accurate. When more refined investigation is 

 taken up some slight changes in the lines of formational delimi- 

 tation may be necessary ; but if the lithologic determinations 



