An effort should always be made to produce a pattern that is subordinate in strength to the main lines of the base map on which it is drawn. In black and white maps, as in colored maps, unlike patterns should be placed next to each other. If they are so placed it may not be necessary to rule the lines on two adjacent areas in opposite directions to produce needed distinctions. A section liner or other ruling device should be used in drawing line patterns in order to produce uniformly even spacing. The application of six of these conventional patterns to a base map is shown in [figure 9].
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Figure 9.—Map bearing six areal line patterns.
STANDARD COLORS FOR GEOLOGIC MAPS.
The standard series of colors for systems of sedimentary rocks is shown on the maps in the Survey's geologic folios but is subject to modifications for use on maps in other Survey reports. Each system is represented by a different color, and if there are two or more formations in one system they are generally distinguished by using different patterns composed of straight parallel lines in the same color. The patterns for subaerial deposits (chiefly Quaternary) are composed of dots or circles, or combinations of both, and may be printed in any color, but the color most often used is yellow or ochraceous orange. No specific colors are prescribed for igneous rocks, but if only a few areas are shown red or pink is preferred. The colors used for igneous rocks are generally more brilliant and purer than those used for sedimentary rocks. For small areas they are used "solid"; for large areas they are reduced in tone by the use of a suitable cross-line pattern or "reticle." Metamorphic rocks are represented by short dashes irregularly placed. These dashes may be in black or in color over a ground tint or over an uncolored area, or they may be in white on a ground tint or pattern. The standard colors used for the sedimentary series covering the 12 systems recognized by the Geological Survey are: Quaternary (Q), ochraceous orange; Tertiary (T), yellow ocher and isabella color; Cretaceous (K), olive-green or rainette-green; Jurassic (J), blue-green or niagara-green; Triassic (
), light peacock-blue or bluish gray-green; Carboniferous (C), blue or columibia-blue; Devonian (D), gray-purple or heliotrope-gray; Silurian (S), purple or argyle-purple; Ordovician (O), red-purple or rocellin-purple; Cambrian (