Subsurface Maps and Illustrations 963 



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

 be reproduced by several different processes, the principal ones being off- 

 set, engraving, lithograph, or metal-lithograph. The line drawings in this 

 chapter are reproduced by the zinc plate, or offset process. 



Shaded drawings, such as figures 499 and 519 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 ex- 

 pensive than black and white line work. Drawings shaded by stippling, 

 ruling, or hachuring, as in the central blocks of figure 498, can be repro- 

 duced 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 

 subsurface 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 quantitative, and any geologic 

 condition that can be reduced to numbers can also be contoured. Ex- 

 amples are thicknesses, elevations, grain sizes, porosities, permeabilities, 

 temperatures, percentages and ratios of any two or more rock constituents 

 or extraneous materials contained in the rocks. In addition to mapping 

 geologic conditions quantitatively it is often necessary to show the chem- 

 ical, mineralogical, or physical characters of the rocks by a means which 

 depicts only the qualitative aspects, as in certain lithofacies maps. It is 

 only when all the properties are mapped that the subsurface geologist 

 can grasp the complete geologic picture. 



The second function of geologic maps is often as important as the 

 first, although it affects the geologist only indirectly. This use is the 

 presentation of mappable geologic phenomena to those having only slight 

 acquaintance with the subject. Often the executives who control the 

 operations of organizations know little about the details of subsurface 

 methods and lack the technical training and experience necessary for an 

 understanding of the geologist's working maps. Since the men who man- 

 age and control the operations of exploratory companies must be kept 



