692 



CHARLES R. KEYES 



must be quite evident that no standard yet proposed for the 

 delimitation of geological units in cartographic representation 

 exactly expresses the essential features of a complete and 

 rational scheme. As in all classifications of natural objects, that 

 of geological units, formations, or terranes should be in its 

 highest type genetic in character. Moreover, it should be strictly 

 stratigraphic, depositional or sequential, using these terms in 

 their broadest sense to include all rock-masses, igneous, meta- 

 morphic and sedimentary. With this understanding of the 

 theme, the most obvious characteristic of a stratigraphic scheme 

 is not the terranes or rock-masses themselves, nor any of their 

 contents, but their geometric elements, their bounding or strati- 

 graphic planes. In order not to carry with it the usual narrow 

 idea, some such term as depositional or sequential planes should 

 be used in place of the name stratigraphic, and these titles, will 

 be hereafter given preference. The sequential, depositional or 

 sedimentation planes have different taxonomic values according 

 to the general scheme of classification adopted 1 . 



A cross-section of the cartographic units, or geological for- 

 mations, as they occur in nature, may be represented by the 

 following sketch : 



Fig. 1. — Geologic units in cross-section. 



In the general, abstract or ideal instance, each lens-shaped 

 figure is the unit established "with regard to all the facts and 

 conditions of the case, and not upon the restricted basis of any 

 part of those facts. It represents as much of the geological 

 development of the earth recorded in the area covered as is 

 practicable." The net-work of formations coincides with the 



1 A??ierican Geologist, Vol. XXIV, p. 294, 1899. 



