722 



WALTER H. BUCHER 



used, permitting the fractures produced on the upper and lower 

 surfaces respectively to interpenetrate. 



Most rock materials, on the other hand, are less brittle than 

 glass and therefore more inclined to yield along shearing planes 

 when subjected to torsion. Moreover, at least in the case of the 

 larger joints observed in nature, the thickness of the formations 

 undergoing deformation through torsion is sufficient to keep the 

 fractures formed on the upper and lower surfaces separate. 



In general, therefore, joints produced by torsion in the course 

 of larger earth movements should occupy the position indicated 

 in Figure 6 with the direction of both, compressive and tensile 

 stress, not differing much from the horizontal. In addition to these, 

 tension fractures, bisecting the angle of the other joints, may occur 

 and even dominate. These, together with an unequal develop- 

 ment of the two sets of shearing joints, with possibly one even 

 missing completely, may give considerable variation to the appear- 

 ance of the same joint system from point to point. 



We may now turn to a discussion of three selected cases of joint 

 systems. 



a) In Figure yyl the general structural relations are given for 

 five points along Crooked Creek, Adams County, Ohio, at which 

 the position of joints was determined by the writer. The joints 

 here cut in a nearly vertical position through the rather thin and 

 even beds of the fine-grained dolomite of the Bisher formation. 



Figure ']B shows the strike of these joints as contained in the 

 following field notes. 



