The following colors and symbols can most appropriately be used to represent ground-water features. The well and spring symbols can be varied by adding letters if they are necessary to express other data than those indicated in the list below. _General ground-water features._ Area of absorption or outcrop: Flat color used on the map to show the geologic system in which the absorbing formation occurs. Areas showing depths to water table: Shades of purple and gray; if possible the shades showing the areas of least depth should be darkest and the shades should grade from those to lighter tints. Contours of water table, or contours on water-bearing formations: Gray or purple curves or lines. Areas of artesian flow: Blue flat tint, or fine ruling in blue. Depth to water-bearing formations: Gradation of a single color or of two related colors from dark for shallow depths to light for greater depths. Nonflowing artesian areas (pumped wells): Green flat tint, or fine ruling in green. Depth to water-bearing formations shown by gradation of tint if possible from dark for shallow depths to light for greater depths. Head of artesian water: Blue curves or lines. Areas that discharge ground water: Blue flat tint, or fine ruling in blue. Areas irrigated with ground water: Green flat tint, or fine ruling in green. [Illustration] Well, character not indicated. [Illustration] Well, nonflowing. [Illustration] Well, flowing. [Illustration] Well, unsuccessful or dry. [Illustration] Well, nonflowing, with pumping plant [Illustration] Well, flowing, with pumping plant. [Illustration] Springs. [Illustration] Spring, thermal. [Illustration] Spring, mineral. The standard color scheme should be used if no conditions preclude its use, but if other colors can be used with greater economy without sacrificing clearness the use of the standard colors should be waived. BLACK-LINE CONVENTIONS. A complete set of the black-line patterns used to distinguish areas on a map is given in Plate VIII (p. 60), and their application to a finished drawing is shown in figure 9 (p. 62). These patterns, however, should preferably not be used by the author in his preliminary work on an illustration. For this purpose water colors or colored crayons are preferable, and the distinctions between areas may be emphasized by letter symbols. MATERIALS USED IN PREPARING MAPS. PAPER. For large and important maps which may at some time be extended to cover a greater area or which may be made to fit maps already prepared or published the paper used should be mounted on muslin to reduce to a minimum the shrinking or stretching caused by atmospheric changes. Pure white paper produces a better negative than a cream or yellowish paper and will retain its color longer, but all papers become more yellow with age and exposure to light.