In preparing an original geologic map a letter symbol, such as has been just described, or a number should be put in the proper place in the explanation, and the same symbol or number should be repeated at one or more places on the map within the areas to which it refers. Each area that is indicated by a color should be marked with the proper symbol in order to make its identification sure, for light colors especially are likely to fade and mixed colors can not be discriminated with certainty. [Illustration: U. S. GEOLOGICAL SURVEY PREPARATION OF ILLUSTRATIONS PLATE II SYMBOLS USED ON GEOLOGIC MAPS, ECONOMIC MAPS AND MINE PLANS] OIL AND GAS SYMBOLS. A complete set of symbols for maps showing oil and gas is given on Plate II. Referring to these symbols the chief geologist, in a memorandum to the Director, writes: The symbols used by the Survey in its oil and gas maps have not been in accord with those used by the oil companies, nor have they been wholly logical. It appears that though they were submitted for recommendation they never have been formally approved. Herewith I submit a code prepared by the geologists of the oil and gas section. They conform largely to commercial use and embrace its best features as well as the best and most logical features of our previous usage, the departures from which are, after all, of minor consequence. The symbols here submitted [see PI. II] with recommendation for approval are founded on a building-up system, so that the history and the results of drilling at any location can be recorded by slight additions to symbol and without erasure. Thus maps may be revised without scratching. In drawing these symbols the draftsman should make the rays of the gas well distinct and in adding the vertical bar or line showing that a hole is dry or abandoned should make it long enough to be distinct. It would be preferable to draw this bar obliquely, but an oblique position would coincide with some of the patterns on certain maps, and it should therefore be placed vertically. The vertical line indicates the failure or abandonment of the well, the symbol for which Is thus scratched off or canceled by the line drawn through it. The symbols agree so far with commercial usage that oil men will have little need to consult the explanation. SYMBOLS FOR USE ON MAPS SHOWING FEATURES OF GROUND WATER. The symbols used on maps relating to ground water represent the features named below, each of which has been shown in publications already issued.