958 Subsurface Geologic Methods 



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 the case 

 might be. 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 



Mississippi an 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 repre- 

 sent 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 interpretation of the lithofacies map by those already fa- 

 milar 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 associa- 

 tions given in figure 509 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 translucent 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 somewhat more dif- 

 ficult 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. 



Several methods of hand coloring are in use, each having peculiarities 

 that are advantageous under certain circumstances. 



Crayon Pencils 



These wax-base colored pencils are applied as evenly as possible over 

 the surface of the sheet, with particular attention being given to boundary 

 margins. When all areas of one color have been covered, a fine, even 



