Subsurface Maps and Illustrations 



961 



indelible colored pencils. The change in tone should be madf^along con- 

 tours at equal thickness intervals. In the figure cited, this interval is 500 

 feet, although the map is contoured on a 100-foot interval. The difference 

 in this type of shading and that employed in shadowgraphic maps is that 

 the latter suggests an illuminated model, whereas the former is shaded 

 strictly on the basis of contour values. 



/ / 





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y^h,-'itt f<,2 ,X" -.„,.y: 



,„i!C,' 'r/'ull^:i:.'ll'X 



Figure 519. Isopach map shaded with pencil to emphasize areas of thick section. 



Reproduction of Maps 



Most maps and other geologic drawings must be reproduced by one 

 means or another. Better prints can be expected if the draftsman is aware 

 of the peculiarities and limitations of the more common processes of re- 



