696 CHARLES R. KEYES 



In considering every unit that is a possible basis for carto- 

 graphic representation, a number of conditions have to be fully 

 satisfied, in order that the best results may be obtained. As 

 nearly as possible the unit adopted should be an abstract one, 

 since schemes which have been elaborated, or may be in the 

 future proposed, may not have different factors or different 

 kind of factors to appose when the new facts are compared. 

 The unit should be practically adaptable in order that knowledge 

 once acquired may not have to be worked over anew in the 

 field with each change of ideas necessitated by the constantly 

 increasing use of more and more refined methods. The unit 

 should be elastic, because too great rigidity of plan often breaks 

 down the best of schemes. The unit should be easily recogni- 

 zable and rapidly delimitable in the field; it should be of such 

 character as to be readily traced from point to point, quickly 

 run in on the map, and easily followed on the ground by sub- 

 sequent investigators who may use the map. 



It has been asserted that the lithologic map is a return to 

 the so-called geological map of a century ago. It does not 

 appear that the facts of the case warrant this statement. The 

 geological map of today based strictly upon lithologic indi- 

 viduals is very nearly as fundamentally distinct from the mineral- 

 ogical map of a hundred years ago, as is the modern map in 

 which so-called geological formations are depicted. In map- 

 ping the geological features of an extensive region, work such 

 as the federal government and some of the state geological 

 surveys are engaged upon, the lithological individual for carto- 

 graphic representation necessarily takes precedence over all other 

 features. It will be along time after the geological map based upon 

 lithology principally is ready to be issued, that the perfected map 

 of ideal geological formations can be made. In the majority of 

 cases the delimitation of the latter must always rest very largely 

 on the lithologic characters. A map of units recognizable in 

 the field only after about as much study as was devoted to the 

 terranes in the first place by the expert stratigrapher is of small 

 practical use. 



For a long time yet in modern areal work, the lithologic indi- 



