Studies for Students 



DEFORMATION OF ROCKS. — IV. CLEAVAGE AND 



FISSILITY {condmicd), JOINTS, FAULTS, 



AUTOCLASTIC ROCKS. 



RELATIONS OF CLEAVAGE AND FISSILITY TO OTHER 

 STRUCTURES. 



RELATIONS OF CLEAVAGE AND FISSILITY TO BEDDING. 



Where a rock series is composed of layers of different litho- 

 logical character, and is in the zone of combined fracture and 

 flowage, the deformation includes the development both of 

 cleavage by normal plastic flow and of fissility in the planes of 

 shearing, in both homogeneous and heterogeneous rocks, and 

 perhaps of all gradations between the two. The beds in the 

 heterogeneous rock may be each approximately homogeneous. 

 There is necessary rearrangement within the beds as well as 

 readjustment between them. Therefore the rearrangement 

 within the beds, in so far as it is not affected by the readjust- 

 ment between the beds, will tend to produce cross cleavage and 

 cross fissility, while the readjustment between the beds will 

 mainly be by parallel slipping, and will tend to produce parallel 

 cleavage and parallel fissility (Fig. lO, p. 478). 



In passing from the limbs of the folds toward the crests or 

 troughs, parallel readjustment becomes less and less important, 

 and normal plastic flow or fracture along the shearing planes 

 becomes more and more important. At the arches and troughs 

 the thrust for a given bed are approximately equal, in opposite 

 directions, and when deeply enough buried its entire thickness 

 is under compression. The direction of least resistance is ver- 

 tical. Therefore the conditions which here prevail are those of 



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