REVIEWS 427 



eludes that in the zone of flow another process is effective to product 

 cleavage; namely, special orientation of minerals and particles. 



Fissility is defined by Van Hise as a parting of a rock along shear- 

 ing planes, which corresponds with the structure described by Becker 

 as cleavage. It is obvious that a capacity to part in a certain plane 

 (Van Hise's cleavage) may often coincide with an actual parting 

 developed later. If the capacity to part be a structure developed in 

 the deep zone of flow, it is most probable that the actual parting 

 should be developed in rising through the zone of fracture, as any 

 rock mass must to become accessible to our observation. Hence 

 arises the opportunity for confusion of the two structures, according to 

 Van Hise, but discriminating the one from the other he concludes that : 

 "Fissility developing in the shearing planes is usually secondary to 

 cleavage which developed in the normal planes." 



The relations of cleavage and fissility to bedding and to folds are 

 discussed at length, and the value of these structures as criteria for 

 determining unconformities are stated and limited. 



With reference to joints Van Hise's discussion is along lines logi- 

 cally consistent with his views of folding and also of fissility. Tension 

 joints may be produced radially to the curvature of folds ; this 

 phenomenon he rightly considers important as a means of accommo- 

 dation to curvature in the zone of fracture. Compression joints 

 develop in shearing planes, usually in two directions, and when 

 associated with simple folds generally parallel to the strike. When 

 folding is complex different sets of shearing plains may correspond to 

 strike-joints and dip-joints, but the association of three or more joint 

 systems may probably be the result of several orogenic movements. 



Faults also are classified as tension faults and compression faults, 

 but recognizing the dominant influence of gravity or of thrust respec- 

 tively, Van Hise well prefers to distinguish them as '"gravity" faults 

 and "thrust" faults. The former is equivalent to the old term 

 " normal," the latter to the inaccurate designation " reversed " fault. 



In discussing the relation of faults to folds it is stated that under 

 moderate load thrust faults may result as clean cut fractures, whereas 

 under greater load they may be associated with overfolds, and at still 

 greater depths overfolds may develop without thrust faults. 



• " This statement might be well supplemented by the further 

 explanation that folds may give place to thrusts on diagonal planes in 

 the deep zone of flowage. Two conditions are essential to folding ; 



