FORMATION THE BASIS FOR GEOLOGIC MAPPING 7^3 



Other things being equal, the determination of the altitude 

 and geographic position of a given point is dependent upon the 

 engineering skill of the observer; and the cartographic unit to 

 be employed does not seriously affect this phase of the work of 

 mapping. With regard to inference, however, this is not the 

 case. No necessary relation exists between a biologic unit and 

 the topography or structure of the area in which it occurs. On 

 the other hand, topography, structure, and the lithologic unit 

 are in general closely related. Recognition of these relations 

 makes it possible to draw inferences concerning the position of 

 the concealed boundary of a lithologic unit from the geologic 

 structure and topography of the area discussed. 



THE RELATIVE UTILITY OF LITHOLOGIC AND BIOLOGIC MAPS 



A lithologic unit is normally also an economic unit, whereas 

 there is no necessary relation between the biologic unit and the 

 economic importance, or lack of importance, of the rocks con- 

 taining a given biota. Though this argument may be regarded 

 by some as of less weight than one based on purely scientific 

 grounds, it is nevertheless valid, as applied to the question under 

 discussion. It should not be forgotten that no geologic survey 

 has ever been instituted, save for economic reasons ; that the 

 chief argument that can be used to obtain state support for such 

 surveys is the direct "economic return to the public ; and that 

 simple justice demands, in return for this support, that the geo- 

 logic results obtained be placed in such a form as to be of the 

 greatest possible use to that public. 



The lithologic unit is also generally related to geographic 

 forms in a very definite manner, as well as to geologic structure. 

 The effects of these relations upon the actual work of mapping 

 have been discussed. Here it is only necessary to point out 

 that both topography and structure are often of economic 

 importance. A map based on a unit which bears some definite 

 relation to them is, therefore, of greater value from an economic 

 standpoint than if the unit be one not so related. 



