another for Mesozoic, and so forth. Subdivisions within a main group are 

 designated by patterns, such as ruling, stippling, and others. Since it is not 

 practicable in most cases to use patterns on hand-colored maps, the color stan- 

 dards set up by the Geological Survey cannot be maintained. However, some 

 consistency can be practiced, and if the colors are well chosen, the result in the 

 finished maps will justify the discrimination exercised in the selections. 



For paleogeologic maps, the coloring is more effective if the darkest tones 

 or most striking hues are used for the oldest rocks, the palest pastel tints being 

 reserved for the youngest systems or formations. As an example, the following 

 associations would be effective: 



Cretaceous . shades of yellow 



Jurassic ..... .... shades of brown, generally medium to light 



Triassic pink, orange 



Permian light red 



Pennsylvanian light and dark grays 



Mississippian shades of blue 



Devonian light green 



Silurian lavender 



Ordovician reddish-purple to bluish-purple 



Cambrian dark greens of different hues 



Pre-Cambrian dark reds, some pattern 



A similar arrangement can be determined when the colors are to represent a 

 number of units within one system. However, an effort should be made to 

 avoid representing a geologic system on one map by a certain color, and on 

 another map by a different color. 



In the different types of lithofacies maps, the main rock types should 

 always be shown by certain colors. It is advisable to follow as closely as possible 

 the color system used to represent lithologies on colored well logs, which, in 

 turn, varies among oil companies. This practice greatly simplifies the interpreta- 

 tion of the lithofacies map by those already familar with the color adaptation in 

 lithologic logging. Some variations are necessary in mapping, particularly where 

 colors of the rocks, as in shales, are important features of the facies; but in 

 general, the associations given in Figure 24-34 are adequate. 



When colors are to be used on the map, some consideration must be given 

 to the kind of prints that are to be made from the line tracing. Thin-paper trans- 

 lucent prints, such as sepias, are not satisfactory. The best prints for coloring 

 are blue-line or black-line Ozalids on medium- to heavy-weight rag-stock paper. 

 Van Dyke positive prints on heavy paper can be hand colored with good results, 

 but they are more difficult to work with than the Ozalids. Photostats are still 

 more difficult because of the natural gloss and hard, impervious surface. Linens 

 are not suitable for coloring. 



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