6 10 STUDIES FOR STUDENTS 



Joints of this class are at right angles to the tensile force. 

 This class of joints is beautifully illustrated in the sharp folds 

 of the graywackes of the Hiwassee river, in the Ocoee series. 

 If the folded rock has planes of weakness of any kind, due 

 either to a primary or secondary structure, the fracture due to 

 the tensile stress may be controlled by these, and thus deviate 

 from the normal planes.^ 



Joints produced by tensile stress may have smooth or rough 

 surfaces, depending upon the character and strength of the rock. 

 If it is a weakly cemented sandstone, the fracture, as pointed 

 out by Becker, is around the grains. If, however, it is a strong, 

 tolerably homogeneous graywacke, quartzite, or limestone, or 

 similar rock, the fractures may be clear-cut and sharp. After 

 joints due to tensile stress have formed, subsequent movements 

 may press the surfaces together, or may fault the strata in a 

 minor or major way, and thus produce slickensided surfaces. 



It has been seen in the discussion of folds that, instead of 

 being simple, and, therefore, in a horizontal attitude, they usually 

 have a pitch ; or, in other words, the rocks are folded in a com- 

 plex manner. In such regions there may be tensile stresses in 

 two directions at right angles to each other, thus producing two 

 intersecting sets of joints. One of these sets, that roughly 

 parallel to the more conspicuous folds, would be called strike 

 joints, while the other set of joints, parallel to the transverse 

 folding, would be called dip joints. Both sets would intersect 

 the bedding nearly at right angles. The fact that two sets of 

 joints in these positions so frequently accord in direction with 

 the strike and dip is strong evidence that many joints are pro- 

 duced by the tensile stress of folding on the stretched half of 



' Becker states that " Tension will not produce joints or cleavages. The theory of 

 the distribution of tension cracks is the explanation of columnar structure " (this Jour- 

 nal, Vol. IV, footnote, p. 444). Where equal contraction occurs in all directions as 

 the result of cooling or desiccation, this is well known to produce columnar forms, 

 but where there is tension in only a single direction, this may produce one set of approxi- 

 mately parallel joints. Tension at a later time in a direction at a large angle to this 

 may produce another set of joints. Or, finally, unequal tension at the same time in 

 two directions nearly at right angles to each other may produce two regular sets of 

 intersecting joints. 



