COLOR SCHEME. 29 



the rocks of middle geologic periods are shown; and (c) if bright greenish or yellow- 

 ish tones prevail, the map indicates rocks of later geologic periods. If the greens 

 are wanting, so that the yellow tones are in contact with blues, he will recognize 

 a hiatus; and similarly if any other color in the sequence is omitted. 



The first and second principles discussed above embody the essential sugges- 

 tions for a general color scheme, but there are other considerations which may be 

 stated as supplementary rules that are at least desirable. Thus, in regard to con- 

 trasts, if we recognize pure hues, tints, shades, and tertiary hues, as explained on a 

 previous page, we may provide that the invariable sequence of colors shall apply 

 equally to pure hues, tints, shades, and tertiary hues; that pure, brilliant hues 

 shall as a rule be reserved for igneous rocks; and that shades and tints belonging 

 to a single minor division of the color sequence shall preferably be so used that 

 darker effects represent the lower and lighter effects the upper terranes, in such 

 manner that gradations from dark to light correspond with conformable sequences 

 and contrasts indicate systemic distinctions, so far as practicable. 



These provisions establish certain usages which have become common — the 

 use of strong, bright effects for igneous rocks as contrasted with subdued effects 

 for sedimentary rocks, and the employment of darker tones for the older and 

 lighter tones for the younger divisions of a group or series. They support the 

 first principle but emphasize contrast. 



Contrast is as necessary to good geologic coloring as harmony. The eye catches 

 a line between contrasted values and identifies the horizon in the various associations 

 that may be occasioned by complex structures. Among the contrasts available 

 for mapping sedimentary rocks, those which occur between light tints of one hue 

 and dark shades of the next hue in the natural order of colors are the most useful 

 and most consistent with harmony. 



With regard to patterns we may recognize that they are desirable in many 

 cases, for they afford distinctions that otherwise may not be clearly made, and if 

 used consistently they serve to suggest at once the igneous, sedimentary, meta- 

 morphic, or surficial character of the rocks. But as patterns may be costly and 

 unless skillfully employed fail in desirable effects, we may agree that the use of 

 patterns shall not be obligatory, yet that if visible patterns are used, it shall be 

 according to an established classification. The experience of the United States 

 Geological Survey indicates that this classification should be patterns in circular 

 figures for surficial rocks, patterns in parallel lines for sedimentary rocks, patterns 

 in hachures for metamorphic rocks, and patterns in angular figures for igneous 

 rocks. Patterns so used must be such as are obvious to the eye at the usual reading 

 distance for which the map is designed. The use of patterns that are not obvious 

 but may be seen on close inspection is' a method of combining colors to economize 

 in printing and may modify the mnemonic suggestion of the origin of the rock. 

 For instance, by close examination of many of the patterns employed for sedi- 

 mentary rocks on the accompanying map of North America, it will be seen that 

 they are made up of crossed lines which would suggest igneous rocks, but these 

 patterns are so fine that they will not be seen at the distance at which the map will 

 ordinarily be read if hung upon the wall. Where conspicuous patterns are employed, 

 either in parallel lines or in hachures, they signify the character of the rock. 



