w- 



Figure 25-4. Stereogram of intersection of two planes. 



To solve the problem by descriptive geometry requires some 12 lines accurately 

 drawn with straight edge and protractor. With the stereographic net, only a 

 piece of tracing paper and a pencil are necessary. 



We shall suppose that plane I dips 30 degrees on azimuth 170 degrees and 

 that plane II dips 60 degrees on azimuth 220 degrees. Figure 25-4 is the stereo- 

 gram of the two planes by cyclographic projection. It is seen that the planes 

 intersect at the center and at a point on the surface of the lower hemisphere. 

 With the stereogram oriented to north, the line o-f gives the direction of the 

 required intersection of azimuth 147 degrees. The position of point e when 

 measured from the circumference of the net along the E-W or N-S line, fixes 

 the plunge of o-e at an angle of 27 degrees. 



535 



