trated, should be used in illustrations requiring considerable details in the 

 horizontal or map plane. Low-angle blocks are more effective where it is desired 

 to emphasize the vertical sections in two directions. The high-angle block may 

 be drawn so that the scale is the same along the two horizontal coordinates; 

 but the low-angle kind should use a somewhat smaller scale along the front-to- 

 back lines in order to produce a more realistic effect of perspective. The scale 

 is reduced along this coordinate in the section lines drawn on the low-angle 

 block in the figure cited. 



Because of the simplicity of the isometric block, it is the one most 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, is somewhat awkward and distorted. Despite this fact, 

 it is the most generally useful of the block diagrams. 



The lower set of blocks in Figure 24-53 is drawn in one-point cabinet 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 vanishing 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 de- 

 parture 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, quadrilaterals. 



The perspective block, although somewhat more diffcult and tedious to 

 draw, is also more natural in appearance. Effects of towering heighs and deep de- 

 pressions and low or high vantage points are readily attained by mechanical draw- 

 ing methods. 



Figure 24-53 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 successively lower, expose more of the upper 

 surface. 



517 



