DEFORM A TION OF ROCKS 1 9 1 



and one set of tensile joints. The compressive joints lie near 

 the longer diagonal of the deformed rectangle, and the tensile 

 joints are near the shorter diagonal (Fig. 5). Ordinarily the 

 former are closer together than the latter. In the third case (i) 

 and (2) are combined ; whether there are two sets of compression 



Fig. 5. Cross-section of a bed showing joints. 



joints, or one is a compression and the other a tension set, will 

 depend upon the relative amounts of shortening and shearing. 



After rupture occurs as the result of tensile or compressive 

 forces, producing one or two sets of joints, if the force in the 

 plane of bedding at right angles to the first direction of force 

 accumulates so as to exceed the ultimate strength of the rock, it 

 may produce other sets of joints. In case this force is tensile, 

 one set of joints will be produced in the normal planes. If it is 

 compressive, any of the three cases of compressive joints above 

 given may occur. Thus there may be produced three or four 

 sets of intersecting joints. 



From the foregoing, the criteria by which tensile and com- 

 pressive joints may be separated are easily inferred. Tensile 

 joints are ordinarily nearly normal to bedding. Compressive 

 joints are ordinarily much inclined to bedding. Between the 

 walls of tensile joints there is ordinarily a small space ; the walls 

 of compressive joints are, or were originally, pressed closel}^ 

 together. The walls of tensile joints are not likely to show 

 differential movements nor slickensided surfaces ; the walls of 

 compressive joints generally show slight differential movements 

 and more or less slickensided surfaces. 



Relations of joints to folds. — Many cases of apparent bending of 

 the strata are really not due to bending but to jointing. It has been 

 j^ointed out that the first is a phenomenon of the zone of flow 



