tails of block diagrams: only the fundamental principles necessary for con- 

 structing the simplest illustrations can be given. 



The two upper blocks in Figure 24-53 are examples of the simplest pro- 

 jection. All opposing sides are parallel to each other, and, because of this 

 feature, they can readily be drawn with a drafting machine or a triangle and 

 straightedge. Distances along the front and back edges and all lines parallel to 

 these edges are drawn to the scale of the map. Distances along the sides and 

 parallel to the sides may or may not be to the 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 illus- 



LOW ANGLE 



HIGH ANGLE 



parallelogram blocks 

 (isometric) 



HOfilZON_ 



3\ 





BLOCKS IN PERSPECTIVE 



Figure 24-53. Geologic block diagrams. 



516 



