l8o STUDIES FOR STUDENTS 



Notwithstanding all of the foregoing difificulties and quali- 

 fications, it is still possible in the field to place most masses of 

 rock somewhat definitely in one of the three zones, the predomi- 

 nant phenomena in most cases of tolerably homogenous rocks 

 being either fracture or flowage, while in heterogeneous rocks 

 of many districts fracture and flowage are both of importance. 



Plastic flow produces folding. — It has been pointed out that the 

 zone of plastic flow is in the zone of folding.' Under the condi- 

 tions of flowage, where the laws of hydrodynamics obtain, there 

 is a constant tendency to approach equilibrium. But because 

 rocks are heterogeneous both in strength and magnitude of ele- 

 ments, this tendency results in very complex flowage, and the 

 resultant forms of deformation include all varieties of rock folds. 

 However, this complexity of flow presents no exceptions to the 

 laws of hydrodynamics.^ At any moment, for any homogeneous 

 small plastic area, for any forces which may be at work, the 

 deformation obeys the laws of hydrodynamics, i. e., the material 

 moves in the direction of least stress. 



Complex folds. — It has been stated that the two sets of simple 

 folds making up complex folds have a tendency to be at right 

 angles to each other. This appears to follow as a necessity from 

 the laws of mechanics. 3 Any number of pressures in all directions 

 may be analyzed into three pressures at right angles tr one 

 another, these being maximum, mean, and minimum pressures. 

 At the outset of the action of the folding forces, because the 

 beds and formations act as transmitters of forces, there will be 

 a tendency for two of the principal directions of stress to be 

 parallel to the bedding, and the other of the principal directions 

 of stress to be normal to the bedding. Even after the layers 

 are inclined it will still be true that at any moment the tangen- 

 tial forces may thus be analyzed. Thus we have the explana- 

 tion of rectangular systems of folds in districts of complex 

 folding. The closer folds are at right angles to the greater 

 horizontal pressure, and the more open cross folds are at right 



^ Loc. cit., (A), p. 594 ; (B), p. 202. 3 Loc. cit., p. 627 (A) ; (B) p. 345. 



^ In my previous articles on deformation by plastic flow I have used the word 

 hydrostatic. The word hydrodynamic should have been used. 



