DEFORM A TION OF ROCKS 6 1 3 



according to Becker, there may be jointing in three or four 

 planes when they are complexly folded, one of these being 

 normal to tensile stress and the others in shearing planes. How- 

 ever, where there are more than two sets of joints at right angles 

 to each other, it is probable that in many cases these have been 

 caused bv successive orogenic movements, the second being in 

 a different direction from the first. Becker has explained that 

 minor faulting is a common phenomenon of compression joints. 



When the folding is simple, both sets of joints developing in 

 the shearing planes, although dipping in different directions, 

 would accord in their outcrop with strike, and might therefore 

 be regarded as strike joints. When the folding is complex it 

 may be that different sets of shearing planes would correspond 

 to strike joints and dip joints, but upon this point further obser- 

 vation is needed. 



In the Knox dolomite of east Tennessee the formation of 

 joints along both tensile and shearing planes is beautifully illus- 

 trated at numerous localities. Commonly the joints produced 

 bv tensile forces are nearly perpendicular to the bedding. Two 

 sets of joints, equally inclined to the bedding and making obtuse 

 angles with each other, are clearly in the shearing planes. 



The attitudes of joints produced by shearing and their rela- 

 tions to bedding would be identical with fissility, as described on 

 pages 593-597. Whether the structure be called fissility or 

 joints would depend upon their number. If numerous and close 

 together the structure would be called fissility ; if fewer and 

 farther apart, jointing. The same compression might produce 

 fissility along one set of shearing planes, and joints along another. 

 If the above be true, it is clear that there are all gradations 

 between joints and fissility. It has been suggested that the term 

 "fissility" might perhaps be wisely restricted to the cases where 

 the structure is secondary to a previous one, such as cleavage or 

 bedding, and that the term "joint" should be used to cover 

 fractures along independent shearing planes. 



In thus explaining many joints as the result of the same 

 forces which produce folds, it is not meant to imply that there 



