DEFORMATION OF ROCKS 605 



tion. The conjunction of these two forces would have driven a 

 flatter dip than would follow from either one of them alone. 



While the above statement is true on the average, the case is 

 complicated because of the differential movements in individual 

 folds. The cleavage for a given section is in a single direction 

 to the southeast only when the folds have a decided monoclinal 

 attitude, and this is especially marked where the folds are all 

 overturned. Even here, however, the cleavage tends to be flatter 

 upon the limbs in normal positions than on the overturned limbs. 

 In the areas in which the folds approach a symmetrical character, 

 cleavage with northwest dips is found on the northwest limbs of 

 the folds. The explanation of these phenomena is given on 

 pages 461-475. 



While there is a general tendenc)- in this region for a south- 

 easterly-dipping cleavage, there are great variations in the steep- 

 ness of the dip in different beds in the same locality, and varia- 

 tions in the average steepness in different localities. The varia- 

 tion in steepness in the same area is explained by the fact that 

 the differential movement between the strata was largely concen- 

 trated in the weaker beds, so that the cleavage in them is flatter 

 than in the more resistant beds. The general variation in the 

 dip of cleavage in passing from area to area may be explained 

 by a difference in the character of the rocks, by a difference in 

 the intensity of the forces at varying depths, or by a difference 

 in the depth of burying. The particular average inclination for 

 a given area depends upon a combination of all these variables. 



With given forces, if the rocks are more resistant in one area 

 than in another, there is less shearing motion, and therefore 

 steeper cleavage in the former than in the latter. Other things 

 being equal, if the forces are more intense near the surface than 

 at a greater depth, the shearing motion is greater at higher 

 horizons, and hence the cleavage is flatter in passing toward the 

 surface. A given force would produce less and less shearing 

 motion with increasing depth, because of the increased friction, 

 and hence the cleavage may be flatter in passing toward the sur- 

 face, just as in the foregoing case. 



