if they are rubbed with drafting pounce or some other very fine abrasive. Of 

 the processes discussed above, only the photstat permits a reduction or enlarge- 

 ment of the original scale. 



Maps and drawings for publication should be drawn to a slightly larger 

 scale than that desired in the reproduction so that the inevitable irregularities 

 occurring in lines, figures, or lettering will be reduced or eliminated in the re- 

 duction of scale. Over-all proportions, however, are not improved in the repro- 

 duction; in some cases, faulty proportions of map features are even more 

 evident in the reduced reproduction. 



Plain black-and-white drawings, that is, black lines on a white or blue 

 background, are the most economical to reproduce. These drawings can be 

 reproduced by several different processes, the principal ones being offset, en- 

 graving, lithograph, or metal lithograph. The line drawings in this chapter are 

 reproduced by the zinc plate or offset process. 



Shaded drawings, such as Figures 24-54 and 24-55 are half-tone reproduc- 

 tions. All reproductions of photographs are made by one type or another of the 

 half-tone process. The half-tone methods are appreciably more expensive than 

 black-and-white line work. Drawings shaded by stippling, ruling, or hachuring, 

 as in the central blocks of Figure 24-53, can be reproduced by black-and-white 

 methods. 



Reproductions in color are expensive and should be avoided when black- 

 and-white or half-tone methods can be substituted. A separate plate must be 

 made for each primary color used. In the printing, the paper must be run 

 through the press for each of the color plates. 



Drafting of Maps 



The effective drafting of geologic maps of all kinds is too important to be 

 neglected entirely, even though the subject cannot be fully discussed here. There 

 are two main reasons for drawing subsurface maps, and which is the more 

 important depends largely upon circumstances. 



The first reason is obvious to those engaged in the technical phases of sub- 

 surface investigations: the various types of subsurface maps are in a sense 

 geological tools. The technician must appraise the various data which he has 

 processed by means of contours so that trends, gradients, anomalies, and other 

 phenomena are developed. Subsurface geology is three-dimensional, and must 

 be developed by a method which takes the three dimensions into account. Only 

 contoured maps can do this in a continuous manner. Contour maps are quanti- 

 tative, a/id any geologic condition that can be reduced to numbers can also be 

 contoured. Examples are thicknesses, elevations, grain sizes, porosities, perme- 

 abilities, temperatures, percentages, and ratios of any two or more rock con- 

 stituents. In addition to mapping geologic conditions quantitatively, it is often 



525 



