DEFORM A rrON OF ROCKS 609 



In ilctcrniininu; unconlorniitics the use of cleavage and fissil- 

 ity follows the same principles as given on pages 351-353 in 

 reference to folding. To safely infer that there is a structural 

 break between series it is necessary to show that cleavage and 

 fnssilitv would be as likely to develo[) in the superior formations 

 as in the inferior ones. It is further necessary that the two 

 series be in actual superposition, not in adjacent lateral posi- 

 tions. 



It has been seen that in proportion as cleavage and fissility 

 develop, the original structures of the rock are obliterated. 

 Where there is apparently complete destruction of the bedding 

 for parts of folds, at areas of less movement, as, for instance, 

 the crests of anticlines and the troughs of synclines, the beds 

 may still be recognized, and thus the relations between the 

 primary and secondary structures be determined. 



JOINTS. 



ORIGIN OF JOINTS. 



Various causes have been assigned for joints, of which the 

 more important are tension, torsion, earthquake shocks, and 

 shearing. It is believed that joints may be classified into ten- 

 sioti joints and compression joints. The first are ordinarily in the 

 normal planes, the second are in the shearing planes. 



Tension joints. — Tension is often due to the contraction caused 

 by cooling or by desiccation. It is well known that the peculiar 

 columnar jointing of igneous rocks is due to the contraction and 

 consequent tension caused by cooling, and the mud cracks of 

 sedimentary rocks are due to the contraction and consequent 

 tension caused by desiccation. However, it is probable that 

 neither cooling nor desiccation is important in the production 

 of systematic sets of joints in the sedimentary rocks. 



It has already been seen that when rocks are simply folded 

 and not too deepl_y buried the convex halves of the anticlines 

 and synclines are subjected to simple tension (pp. 205-208, Fig. 

 i). If the tension goes beyond the limit of elasticit}^ radial 

 cracks will be formed which strike parallel with the rocks. 



