THE MECHANICAL INTERPRETATION OF JOINTS 715 



b) It is easy to see that the joints on Mme Fork, Kentucky, 

 described in the introduction to this paper, correspond to this 

 type. Here, however, the active stress was the horizontal tension 

 existing at the top of the anticline, while the weight of the 

 overlying rock-masses, giving the compressive stress, was merely 

 passive. 



The analysis of joints can, however, be carried farther and 

 may often yield information of decisive value to the field geologist. 



Fig. 3. — Diagram illustrating the position of the planes of shearing in a brittle 

 body subjected simultaneously to vertical compression and horitonzal tension. 



A detailed analysis of the joints on Mine Fork will be given here 

 to illustrate the method of analysis used by the writer. 



The exposure on Mine Fork is such as not to give the true dip 

 of either of the joint planes. The joints themselves are filled with 

 mineral matter and their surface is nowhere exposed. But their 

 apparent hade was measured on the vertical face of the exposure 

 trending essentially NNE — SSW and their strike was determined 

 on the level top of the cliff. 



Apparent hade: 

 Strike: 



Set I 



27° northward 



N 78W 



Set II 



N27W 



