The geometric centers of the faces are shown in several instances at the 

 intersections of the diagonals. It is quite apparent that the center of the block 

 is always to the rear of the scaled midpoint. This is illustrated in the sectionized 

 block on the left where the spacing between section lines is progressively less 

 from front to back. This fact must be kept in mind when geologic features are 

 transferred from maps to blocks in one-point perspective. 



The geologic diagrams at the bottom of the figure illustrate the use of 

 secondary vanishing points. The block on the left shows an anticline and two 

 synclines, the axes of which are parallel and trend diagonally across the block. 

 Since the block is drawn in perspective, these parallel lines must also be shown 

 with the same degree of convergence; therefore, it is necessary to select a new 

 vanishing point on the horizon (2 in the figure) . When the folds are constructed 

 according to the convergence of lines into this point, the perspective in the 

 geologic features will be the same as that in the block. 



The lower-right diagram utilizes three vanishing points, as indicated by 

 the construction lines. Point 3 is the focus for the lines bounding the fault plane, 

 and 2 controls the lines cutting off the corner of the block. 



Figure 24-54 shows a structural map and one-point perspective block of 

 the same area. Both have been shaded with a pencil to emphasize the structural 

 relief. A contour map shaded in this manner is called a shadow-graphic map. 

 There are two methods for attaining the shadow effect. The simplest is by hand 

 shading, as mentioned. In both the map and block it is assumed that the source 

 of light is the upper left-hand corner. All structural surfaces facing this corner 

 receive the greatest amount of light, and those sloping toward the lower right- 

 hand corner bear the heaviest shading. High points have high lighting; low 

 areas, dark shadows. 



A more cumbersome method consists in first soaking a contoured map until 

 it can be moulded into ridges and depressions conforming to features shown by 

 the contours. This may be done by working the softened map over a mass of 

 wet papier-mache. When the modeling is completed, the shadow-graphic map is 

 obtained by photographing from directly above with one source of light, pref- 

 errably from the upper left-hand corner. Obviously, this is a much more tedious 

 method than shading the map with a pencil. 



Coloring of Geologic Maps 



Many kinds of maps require coloring, and since the maps that are made 

 by commercial organizations are not reproduced in color by printing proceses, 

 they must be colored by hand. When several copies of a map are needed, the 

 coloring of the prints may develop into a burdensome task. For this reason, it 

 is desirable to be familiar with a number of different methods in order to select 

 the one which will best satisfy the needs of the project. 



518 



