REPRESENTATION OF GEOLOGICAL FORMATIONS 697 



vidual, delimited if necessary by the aid of other criteria, 

 appears amply comprehensive and exact for all purposes to 

 which the ordinary geological map is put. As an aid in the 

 development of the mineral resources of the country — the 

 primary object of work of this kind — maps in which the litho- 

 logic individual is the unit amply suffice. In practice in the 

 field, the units broadly defined by the lithologic characters, and 

 those indicated by the more philosophical geologic formations, 

 are generally near enough alike to enable future investigators to 

 do their work without hindrance or uncertainty. 



The suggestion of a faunal map eventually following the 

 lithologic map as an integral part of a complete geologic 

 atlas appears somewhat infelicitous. There is certainly no 

 room whatever for such a dual plan in mapping. Such a scheme 

 merely leads to others, maps based upon every criterion known 

 or which may be devised. This is a proposition for which there 

 is not the slightest demand. It is beyond all probability that 

 parallel subdivisions should ever be found that are based upon 

 radically different criteria. When we consider a dual scheme 

 with a structural phase and a time phase based entirely upon 

 fossils we are considering incongruous things. And there is 

 not necessarily any logical connection. 



In Europe, there is a classification generally presented that 

 is dual in character, though with singular nomenclature. Thus, 

 all the subdivisions of terranes and of time are strictly paralleled. 

 The International Geological Congresses have adopted the same 

 plan. It must be quite evident to the practical field geologist 

 that there are very serious objections to this scheme. The 

 critical criteria in the rock-scheme and in the time-scheme are 

 fundamentally distinct. In fact, they have no genetic relation- 

 ships whatever. 



In reality, we have more than a dual scheme of classification 

 in geology. There is a triple scheme, a quadruple scheme, and 

 schemes multiple according to the number of standards involved. 

 Each standard gives rise to a different scheme. 



The principle underlying the classification of natural phe- 

 nomena is that different kinds of criteria give rise to different 



