Methods employed 72 Photoengraving 72 General features 72 Zinc etching 73 Copper etching in relief 75 Half-tone engraving 75 Three-color half-tone process 78 Wax engraving (the cerotype process) 80 Wood engraving 81 Photogelatin processes 82 Lithography 83 Original process 83 Photolithography 86 Offset printing 87 Chromolithography 87 Engraving on stone and on copper 89 Appendix. Length of degrees of latitude and longitude 91 Metric system and equivalents 92 Geologic eras, periods, systems, epochs, and series 92 Chemical elements and symbols 93 Greek alphabet 93 Roman numerals 93 Mathematical signs 94 Names of rocks 94 ILLUSTRATIONS. Page. Plate I. Methods of inserting plates and figures 10 II. Symbols used on geologic maps, economic maps, and mine plans 20 III. Lithologic symbols used in structure and columnar sections to represent different kinds of rock 32 IV. Symbols used on base maps 52 V. Reduction sheet used in lettering illustrations 54 VI. Half-tone prints showing effects produced by the use of six standard screens 56 VII. Details of the make-up of a geologic map 58 VIII. Patterns used to show distinctions between areas on black and white maps 60 IX. Diagrams and curves 64 Figure 1. Diagrams showing principal, guide, and auxiliary meridians, standard and special parallels and correction lines, and system of numbering townships, ranges, and sections 16 2. Conventional lines used in preparing plans and diagrams of mine workings to distinguish different levels 29