6o6 STUDIES FOR STUDENTS 



For any given area, after cleavage developed, as denudation 

 progressed the zone of flowage passed upward into the zone of 

 fracture. It is clear that the cleavage planes already developed 

 were then probably shearing planes, and this was true even if the 

 horizontal thrust was the same and in the same direction in the 

 zone of fracture that it was in the zone of flowage, for the direc- 

 tion of greatest normal pressure is composed of thrust and 

 gravity, and therefore at a great depth is steeply inclined to the 

 horizon, whereas in the zone of fracture, gravity being less impor- 

 tant, the direction of greatest normal pressure is less steeply 

 inclined, and therefore normal planes in the zone of cleavage 

 become shearing planes in the zone of fracture. In the develop- 

 ment of fissility along the cleavage planes there were slight 

 differential movements between the laminae, and hence was formed 

 the very extensive fault-slip cleavage so well known in the 

 Appalachians. It is believed that the more regular and wide- 

 spread fissility is thus secondary to cleavage, but it is recognized 

 that fissility or joints formed in other directions, and that in the 

 outer zone, which was never in the zone of flowage, original 

 fissility or jointing only was developed. 



As pointed out by Willis, in the western area of little altered 

 rocks in the southern Appalachians the deformation was mainly 

 by faulting ; in the corresponding area in the northern Appala- 

 chians the deformation was mainly by folding. At intermediate 

 areas the deformation was by faulting and folding. Parallel to 

 the fault-planes fissility developed to some extent in the area of 

 fracture, and dipping in the same direction as the monoclinal 

 folds cleavage developed in the area of folding. In an inter- 

 mediate area the deformation was by major faulting, by minor 

 fault-slips along fissility, by the pure shortening and shearing 

 motion producing cleavage, and by monoclinal folds, all com- 

 bined. The interactions of these are more fully described in 

 other places (see pp. 595-598,620-622). 



Returning to the crystalline area, in the cracks and crevices 

 between the fissile laminae mineral impregnation from water 

 solution occurred at many places, and thus gave the rocks a 



