DEFORM A TIUN OF ROCKS 597 



RELATIONS OF CLEAVAGE AND FISSILITY TO THRUST FAULTS. 



Between cleavage and fissility developed along the longer 

 limbs of folds and thrust faults, which accord in dip with the 

 beds on the longer limbs, there is only a difference in the 

 magnitude and frequency of the movements. 



When fissility or cleavage develops there are many slightly 

 or infinitesimally separated movements of small degree. When 

 a thrust fault develops there is a single major movement. It is 

 believed that the relief is more likely to be by faulting at little 

 de|)th, and at greater depth is more likely to be by the 

 development of cleavage, and often, secondary to it, fissility, 

 or more rarely by the development of fissility directly. The 

 passage of cleavage by gradation into minute overthrust faults 

 is beautifully illustrated a short distance northwest of Blowing 

 Rock, N. C. Where fissility is not developed throughout the 

 rock mass it may occur adjacent to thrust faults, due to the 

 shearing adjacent to the thrust planes. Rock masses deformed 

 bv thrust faults and showing fissility adjacent to the faults will 

 ha\e zones of fissility which alternate with others in which this 

 structure is absent. Where fissility varies in perfection of devel- 

 opment in alternating zones, but is present throughout the 

 rock mass, it implies that the relative movements were con- 

 centrated to a certain extent along definite zones, but that move- 

 ment everywhere occurred. It is plain that shearing developing 

 cleavage and fault slips alo7ig planes of fissility accomplish the same 

 mass deformation as do thrust faidts, oViXy it is averaged through- 

 out the rocks instead of being largely concentrated at certain 

 planes. 



Differential movements similar to those described in the 

 above paragraph may. occur along a lamellar structure in any 

 kind of a rock, sedimentary or igneous. 



By differential movements, such as are above described, 

 enormous masses of material ma)- move forward long distances 

 The top of the mass, having the advantage of all the differential 

 movements below, will travel the farthest (Figs. 5 and 6, p. 468). 

 The base will move the least. P'rom this mass at some later time 



