DEFORM.] TION OF ROCKS 



617 



tion into tension faults and compression faults ; for it is possible, 

 though hardly probable, that an inclined fracture may result from 

 com[)ression, and after a time thrust lessen in amount, so that 

 gravity controls the final differential movement. 



Fig. 15. — Reverse or thrust fault. 



RELATIONS OF FAULTS TO EXPANSION AND CONTRACTION. 



Gravity faults result in the dilation of the part of the crust 

 affected by them ( Fig. 14). Thrust faults result in the contraction 

 of the part of the crust affected by them (Fig. 15). In a region 

 in which many parallel faults occur, all of the same character, 

 the dilation, or contraction may be a considerable percentage of 

 the breadth of the area disturbed. Since the amount of dilation 

 or contraction with a given vertical movement increases as the 

 hades become flat, and since thrust faults have flatter hades on 

 the average than gravity faults, the shortening of the crust in a 

 region of thrust faults is usually greater than the elongation in 

 another region in which gravity faults are about equally abun- 

 dant and in which the vertical displacements are the same. 



RELATIONS OF FAULTS TO STRIKE AND DIP. 



While there is great variability in the direction of faults, due 

 to exceptional causes, faults are more parallel to the strikes and 

 to the dips, other things being equal, than in other directions, so 

 that faults are sometimes spoken of as strike faults and dip faults. 



