THE MECHANICAL INTERPRETATION OF JOINTS 717 



4. Find the point i, located halfway between e and e' . The 

 line iO, lying in a plane normal to the intermediate stress dO, 

 and in the plane hidj bisecting the acute angle of the shearing 



Fig. 4. — Stereographic projection of the joints on Mine Fork, Kentuckj'. Ex = 

 Trend of exposure ; a& = set I of joints; a'i' = set II of joints; 0(f = line of intersection 

 of joint planes = position of intermediate principal stress; Atrf;'= plane bisecting the 

 acute angle of the joint planes; oJ = located in this plane, normal to orf = position of 

 the greatest (compressive) principal stress; W(f/ = plane bisecting the obtuse angle of 

 the joint hid planes; og = located in the plane mdl, normal to o<i = least (tensile) prin- 

 cipal stress; mdl, g^/= principal planes. 



planes, represents the direction in space of the compressive stress . 

 The line hj gives the trend of this stress in a horizontal plane. 



5. go° from i, on the great circle gif, mark the point k, which 

 in this case practically coincides with /. The line kO, lying in 



