934 



Subsurface Geologic Methods 



scale of the map; but, in any case, the scale is constant along these lines. 

 This type of block is sometimes called a "parallelogram block," but it is 

 essentially an isometric projection. The block may be drawn with any 

 desired degree of tilt. The high-angle block, as illustrated, 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 em- 

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

 be drawn so that the scale is the same along the two horizontal coordi- 

 nates; 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 



HIGH ANGLE 



LOW ANGLE 



parallelogram blocks 

 (isometric) 



-«OE!E3^^^^^ 



HOf!/ZON_ 



•.^ VANISHING 

 - , ~-, POINT 



BLOCKS IN PERSPECTIVE 



Figure 498. Geologic block diagrams. 



