Colored pencils and crayons are useful only for coloring preliminary maps. They are not recommended for use on maps that are to be kept for reference or to be submitted for reproduction, because the colors rub off, but they can be used on photographic prints of base maps or on transparent oversheets, for which the unglazed side of tracing cloth is well suited. When they are so used register marks should be added at numerous points on the map and the oversheet, including the four comers, the color boundaries should be drawn or traced, and finally the colors should be added. Two or more colors should not be used on any one area to modify a tone, but each area should be colored with a separate crayon. Patterns or designs should not be used except to strengthen contrasts, and for that purpose a pattern may be drawn with a black pencil over a color. WATER COLORS. By dilution to half strength some of the standard water colors will yield a tint or hue that will contrast with other tints or hues produced in the same way quits as well as undiluted or full colors will contrast with one another. The colors named below, except chrome-yellow and emerald-green, are among those that when diluted will afford satisfactory contrasts among themselves and with their full colors and are recommended for use in coloring original maps. Mauve. Hooker's green No. 2. Crimson lake. Emerald-green. Orange-vermilion. Payne's gray. Burnt sienna. Lampblack. Cadmium-yellow. Sepia. Chrome-yellow. Cerulean blue. Olive-green. Other pigments spread better than cerulean blue and emerald-green, but the exceptional purity of color of these two seems to warrant their use. JAPANESE TRANSPARENT WATER COLORS. Japanese transparent water colors, so called, are used by some geologists. They spread evenly and are convenient for field use, but they can not be washed out like other water colors, so that when they are once applied to an area and a change of color becomes necessary they must be bleached out. A good bleach is sodium hypochlorite, which should be applied with a brush until the color disappears, and the area dried with a blotter before recoloring. Light tints of these colors are believed to be somewhat fugitive if exposed to strong light. COLORING GEOLOGIC MAPS.