Several processes are used for preparing plates for printing illustrations, and each has its peculiar features of excellence. One process may render fine details with facility but may fail in uniformity in large editions; another may be cheap and effective on the whole but may not reproduce fine details; and still another may give fine color or tone effects but may be too expensive. Therefore a knowledge of the varied uses and results and of the cost of the several processes of reproduction and, on the other hand, of the kinds of originals that are best suited for reproduction by any one of the processes is essential to effectiveness and economy in planning, preparing, and reproducing an illustration. The following condensed descriptions of processes are intended mainly to aid in determining the kind of copy that is appropriate for each process and the kind and quality of reproduction to be expected, so that only the principal operations or stages in each process are described. Wood engraving, which was used in making printing plates for many of the illustrations in the early publications of the Geological Survey, is described here only to compare that laborious and "indirect" method of engraving cuts with the more modern kinds of relief engraving. In 1892 it gave way to photo-engraving. PHOTO-ENGRAVING. GENERAL FEATURES The term "photo-engraving" is applied to processes by which a black and white line drawing, photograph, or like original is reproduced in relief on a metal plate from which prints may be made on an ordinary printing press, in distinction from processes that print from flat or relatively flat surfaces, such as the lithographic and photogelatin processes. The photo-engraving processes that are most generally used are those called "zinc etching" and "half-tone engraving." These processes depend on the discovery that gelatin or similar organic material, if treated with potassium or ammonium bichromate and exposed to the action of light, is made insoluble in water. If a metal plate coated with bichromatized gelatin or albumen is exposed to light under a negative the parts acted upon by light become insoluble and those not acted upon remain unchanged and may be washed away so as to expose the metal, which is then etched with acid in order to give relief to the unexposed parts and make of them a printing surface. ZINC ETCHING.