Subsurface Maps and Illustrations 935 



commonly used in geologic illustrations. A number of examples appear 

 in this chapter to demonstrate various features. Since this type of block is 

 not a true perspective figure, the distant or upper end appears to be larger 

 than the front or lower end, and the block, therefore, appears somewhat 

 awkward and distorted. Despite this fact, it is the most generally useful 

 of the block diagrams. 



The lower set of blocks in figure 498 is drawn in one-point per- 

 spective. In this construction, lines forming the sides of the block and 

 all others parallel to these converge into a single point called the "vanish- 

 ing point." The vanishing point lies on the horizon, which, in turn, is 

 level with the observer's viewpoint. Any pair of parallel lines not parallel 

 to the construction lines of the block also converge to a point on the 

 horizon but to the right or left of the vanishing point of the block. The 

 blocks shown in the figure present one face without perspective distortion 

 directly toward the observer. This is a departure from a true or natural 

 perspective drawing, except in the one case where the vanishing point 

 lies directly above or below the block. Such a view does not expose the 

 sides of the block. All other positions of the block would, in natural 

 perspective, require some convergence in the frontal face; but since this 

 would unnecessarily complicate the drawing of geologic features, the 

 front of the block is made a true rectangle, and the sides and top, quadri- 

 laterals. 



The perspective block, although somewhat more difficult and tedious to 

 draw, is also more natural in appearance. Effects of towering heights and 

 deep depressions and low or high vantage points are readily attained by 

 mechanical drawing methods. 



Figure 498 shows blocks in various positions relative to the observer. 

 The uppermost blocks are above the horizon and, therefore, above the 

 observer's position. The block in the upper left is placed so that the bottom 

 is exposed to view. The stack of three blocks on the right is drawn so 

 that the base of the stack is somewhat below the observer's eyes, and the 

 top at a considerable height above. Note that the second block in the 

 left-hand series lies on the horizon, and, consequently, the eyes are exactly 

 in the plane of the upper surface. The two blocks below, which are suc- 

 cessively lower, expose more of the upper surface. 



The optical 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 



