Maps are usually colored for practical reasons, not merely for embellish- 

 ment. The main purpose in using colors is to increase the legibility so that sig- 

 nificant features can be seen at a glance. Because of this fact, color contrasts are 

 more desirable than color harmony with low contrasts of tone or hue. On the 

 other hand, clashing colors should be avoided if pleasing effects are desired. 



The U. S. Geological Survey and various state surveys use only certain 

 colors for rocks of specific geologic ages; one color range for Paleozoic rocks, 



N~ 



/' 



A 



Figure 24-54. A — Shadow-graphic structure map. B — One-point 

 perspective block diagram. 



519 



