necessary to show the chemical, mineralogical, or physical characters of the rocks 

 by a means which depicts only the qualitative aspects, as in certain lithofacies 

 maps. It is only when all the properties are mapped that the subsurface geologist 

 can grasp the complete geologic picture. 



The second function of geologic maps, which is often as important as the 

 first, although it affects the geologist only indirectly, is the presentation of map- 

 pable geologic phenomena to those having only slight acquaintance with the 

 subject. Often the executives who control the operations of organizations know 

 little about the details of subsurface methods and lack the technical training 

 and experience necessary for an understanding of the geologist's working maps. 

 Since the men who manage and control the operations of exploratory companies 

 must be kept informed on the geologic developments, it devolves upon the geolo- 

 gist to construct maps that convey the essence of his work in a direct manner, 

 without the details so necessary in the working maps. Maps that are to be used 

 for the purposes described above should emphasize the ideas and conclusions of 

 the geologist, not merely present or evaluate geologic data. 



The methods of coloring maps have been described. But other features of 

 map construction are equally important. It is difficult to show a number of 

 different classes of geologic data on one sheet without some confusion of lines 

 or areas. Therefore, the legibility of the map should be the principal guide as to 

 how much can be shown to advantage. Several factors influence the legibility 

 of a map: 



(1) Standard symbols — The U. S. Geological Survey as published sheets of 

 standard symbols for geologic maps. These symbols should be used wherever 

 applicable because they are more likely to be understood by everyone using the 

 map. If it is necessary to invent a symbol to indicate some subsurface feature, 

 this sheet of standard symbols should be consulted to avoid using a figure 

 which is standard for some other feature. The size, form, and weight of symbols 

 should be kept uniform, except in special cases where a variation in the size or 

 mass of the symbol denotes a corresponding variation in the size or importance 

 of the feature shown. 



(2) Line weights — The careful grading and uniformity of line weights have 

 much to do with the legibility and general appearance of the map ; for example, 

 line weights should grade downward from state boundaries, county boundaries, 

 townships, sections, etc., despite the fact that certain of these are further dis- 

 tinguished by various sequences of short and long dashes. 



(3) Lettering — The lettering on most maps is done largely by LeRoy or 

 Wrico lettering sets, which produce letters in a wide variety of sizes and line 

 weights in both slanting and vertical styles. The presentation of the map is 

 greatly enhanced when the choice of letter sizes and weights is made judiciously. 

 Before any lettering is done, the features should be classified, and proper templets 

 selected for each. The same templet will produce different-appearing letters when 



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