DEFORMATION OF ROCKS 595 



bedded rocks. The different layers may be due to secondary 

 structures. They may be due to the flowage of igneous material 

 along primary or secondary planes of weakness, or to secondary 

 water-deposited impregnations along such planes. They may be 

 due to heterogeneous injections. In all of these cases, as in any 

 other, the stronger beds will to a certain extent control 

 the movements. The accommodations will occur along the 

 layers when folded, and there will be a tendency for cleavage 

 and fissility to develop parallel to them. 



The more unequal the layers in thickness and strength, the 

 more likely is the major accommodation to take place parallel 

 to them, and thus produce cleavage and fissility which nearly 

 accord with them. In proportion as the rocks approach massive 

 ones, the law of cross structures prevails, but in a minor way 

 readjustments along the laminae may occur, and these laminse 

 still retain their integrit3\ In rocks as they occur in the field 

 both tendencies are always present in all the parts, from the 

 minute laminae to the largest masses. Sometimes the first is 

 predominant, sometimes the second. If the lamination is not 

 strongly marked the first tendency will control, although the 

 lesser la3'ers or laminae of unequal strength may in a minor way 

 control the movements. 



Ordinary shale is a representative of rocks having minute 

 layers of slightly different strength. Usually the average of the 

 cleavage or fissility distinctly cuts the beds. The material yields 

 to thrust by flowage or fault slips combined with minute pucker- 

 ings. If the process be continued far enough the individual 

 layers are folded upon themselves in a large number of parallel 

 folds, nearlv at right angles to, or inclined to, their original 

 positions. Therefore, while cleavage does nearly correspond 

 with the original beds, the particles have been so much deformed 

 and rearranged and the layers have been so far readjusted as to 

 make the term bedding scarcely applicable. 



As has been seen (p. 474) , beds are particularly likely to 

 largely control the direction of cleavage or fissility if the folds 

 are monoclinal and overturned. In such folds the major dif- 



