PREPARATION OF A GEOLOGIC MAP 65 
Colors and conventions—Whenever possible, the colors and 
patterns adopted by the United States Geological Survey should 
be adopted, for the sake of uniformity. Whether this be done 
or not, the maps issued by any one survey should be treated 
according to some one system. This is especially necessary 
when the maps so issued form a series, as in the publication of 
a number of county maps. 
A formation should never be expressed by entire lack of 
color and pattern, as such a practice frequently leads to con- 
fusion. This, of course, refers particularly to black-and-white 
maps, as formations are rarely so treated on colored maps. 
In small or local maps, particularly in such as are prepared 
for general use or to illustrate the distribution of economically 
important formations, the colors or conventions used for the 
different formations should be as distinct as possible. The 
smaller the areas to be distinguished, the greater will be the 
necessity for careful choice of colors or conventions, in order to 
obtain the greatest possible contrast. 
In maps either of large scale, or covering large areas (in 
which case the geology will be of necessity much generalized), 
the general effect of the color scheme should be considered. In 
the allotment of colors and conventions to the different forma- 
tions, it should be borne in mind that, the smaller the areas 
covered by any one formation, the brighter the color and heavier 
the pattern it will bear without seeming crude or obtrusive. As 
large maps are, in general, mounted and used as wall maps, a 
formation occurring mainly or entirely near the top of the map 
may be given a brighter color or more striking pattern than one 
in the middle or near the bottom of the map. 
Patterns.—If patterns be used, with or without color, certain 
additional points must be considered. In the case of a forma- 
tion occupying small areas, the pattern selected for that forma- 
tion should bear some definite relation to the form and orientation 
of those areas. For example, a formation extended as a narrow 
belt should be given a pattern whose ruling is, as near as pos- 
sible, at right angles to the general trend of the belt. 
