698 CHARLES R. KEYES 



taxonomic groups. The arrangement of rock-masses affords no 

 exception to the rule. If the life phases be used as the pre- 

 dominant feature in delimiting the subdivisions of one order, 

 prevailing lithologic character may be given greatest weight in 

 another; physiography or specific biotic aspect in a third. 

 Matters are greatly simplified by regarding the larger subdivis- 

 ions of the geologic scale as essentially arbitrary, abstract time 

 divisions, in which lithology has no place. With the smaller 

 subdivisions, which are best considered as essentially structural 

 divisions, the time element may be practically neglected. 



While we have to allude to the time-interval, during which 

 every rock-mass was formed, we have only universal structural 

 units represented in the major two of the five taxonomic cate- 

 gories usually recognized. On the map, the expression of the 

 time of formation of the terrane is by a distinctive color. The 

 smaller of the universal time-units is thus represented. In like 

 manner, only one of the rock categories becomes important on 

 the map, and this is commonly represented by a standard pat- 

 tern. This, too, is the smallest unit that is of broad geologic 

 significance. While this plan does not always meet every case, 

 it only needs slight adjustment from time to time in order to make 

 it the most serviceable, the most practical, the most elastic, and 

 most nearly in accord with local facts, of any scheme yet devised. 



After all, the cartographic unit, like the species in zoology 

 or botany, is necessarily a matter of convenience. In the exact 

 delimitation of both, there enters very largely an element of 

 personal judgment. Through the consensus of opinion we finally 

 arrive at a tolerably good idea of what each unit should be. 



It is commonly recognized that the principal criteria followed 

 in delimiting the several taxonomic orders of units and in geo- 

 logical classification are (1) the relative progress of life in 

 general as compared with that now existing; (2) the prevailing 

 biotic type ; (3) the general lithologic phase; (4) the specific 

 lithologic character; and (5) the specific fossil feature. 



Reference is here made to our most approved ideal classi- 

 fication because the lithologic individual fits closely into this 

 scheme as the taxonomic subdivision of the fourth order. The 



