;i4 



PRINCIPLES OF STRATIGRAPHY 



and with reference to the force producing them they may be 

 classified as gravity and thrust faults. 



With reference to direction. The classification with refecence to 

 direction has already been in part explained. It includes strike 

 faults, dip faults and oblique faults, with reference to the strata. 

 With reference to each other, they may be parallel, intersecting or 

 radial, the last when radiating roughly from a point. With refer- 

 ence to a geological region whether raised or depressed (geologi- 

 cally, but not necessarily topographically), they may be peripheral, 

 when running along the periphery of the geological formation, or 

 cross-faults when they cut across it. 



With reference to uwvcinent. Movement is relative. If the 

 stratum of the hanging wall in any given section is lower, it is 

 designated a normal fault. If the stratum of the hanging wall is 

 higher, it is a reverse fault. 



Fig. 196. 



With reference to cause. In most cases normal faults probably 

 mean a tension and a down-sinking of the hanging wall, accom- 

 panied by an elongation of that part of the crust. In this case the 

 normal fault would also be a gravity fault. In like manner the re- 

 verse fault generally implies an upward movement of the hanging 

 wall with consequent foreshortening of the crust by overlapping 

 under pressure. In such ^ case the reverse fault is also a thrust 

 fault. Under certain conditions, however, a gravity fault may in 

 section show the conditions of a reverse fault (Figs. 196, A B), for, 

 though the hanging wall has slipped down, owing to the oblique 

 character of the displacement and the dip of the strata, the beds 

 seem actually to have slipped up over each other. 



In like manner a thrust fault may in section show the charac- 

 ters of a normal fault, the ends of the strata on the hanging wall 

 being actually lower than on the foot wall. This is shown in Figs. 

 197, A B. 



