DEFORMATION OF ROCKS 623 



comj^licatcd faulting than an upper series which lithologically 

 is cciually likely to be faulted, this is strong evidence that 

 the lower set of formations was faulted before the upper set of 

 formations was deposited. Faults are frequently not easy to 

 demonstrate or to trace out. Hence this criterion for discrim- 

 inating between series is not so valuable upon the whole as are 

 the more conspicuous and readily discovered phenomena of 

 folds, cleavage, fissility, and joints, but if the conditions are 

 favorable for tracing out the faults of a region, the informa- 

 tion thus furnished may give positive evidence as to structural 

 breaks between series. 



GENERAL. 



.In the foregoing pages folds, cleavage, fissility, joints, and 

 faults are regarded as the conjoint products of thrust and 

 gravity. Similar forces acting upon heterogeneous rocks under 

 various conditions produce diverse phenom.ena. Thus several 

 classes of phenomena which are often treated as independent 

 and unconnected are genetically connected. A fault may 

 accord in inclination with any of these structures. Between 

 faults and joints, fissility, or cleavage there are all gradations. 

 When there is a marked displacement along a break it is called 

 a fault. Whether a given minor displacement is thus named 

 often depends upon the point of view. Wherever there is fissil- 

 ity there is slipping or faulting, using this term in its exact 

 sense. Usually minor displacements are not called faults unless 

 they occur across the beds or other structures. A displace- 

 ment across a prior structure of such magnitude as to be called 

 a fault might be ignored if it occurred along the structure. 

 Folding and cleavage belong normally to the zone of flow- 

 ing ; fissility, joints, and faults belong normally in the zone of 

 fracture. In the zone of combined flowage and fracture all 

 the structures occur together in a complex manner, the par- 

 ticular combination of phenomena depending upon the relative 

 thickness, strength, and brittleness of the rock beds con- 

 cerned. 



