FISSURES, OR FRACTURES. 



231 



sequent erosion, so that the unpractised eye detects nothing unusual 

 along the line of fracture and slip. In Fig. 198 the strong line a a 

 shows the present surface, while the dotted line bbb shows the surface 

 after the displacement as it would be if unaffected by erosion. In 

 many cases, however, it seems more probable that there never existed 

 any such escarpment as represented in Fig. 198, but that the displace- 



Fig. 200. — Section through Portion of Plateau Pegion of Utah, showing a Succession of Faults 



(after Howell). 



ment was produced by a sloiv, creeping motion, or else by a succession 

 of smaller sudden slips probably accompanied with earthquakes (p. 

 113), and thus that the slipping and the denudation have gone on to- 

 gether pari passu. In Fig. 227, on page 255, the upper part shows the 

 great Uintah fault 

 restored, while the 

 lower part shows the 

 actual condition of 

 things produced by 



erosion. 



When faults 



Fig. 201. 



oc _ j.xu. ,«,.!.— Fault with Change of Dip : d, dike. 



cur in inclined outcropping strata, the same series of strata may be re- 

 peated several times, as in Fig. 199. In such a case, the observer walk- 

 ing over the surface of the country from A to B might suppose here a 



series of nine strata, whereas there are 

 ':;.•.•'.■•':::•.•••■.■■ ■;': \ : ^^\-".7 •-/•': •;:/.■ ;. but three strata, a, b, c, three times re- 

 peated. Fig. 200 is a natural section 

 showing this. Sometimes the dip of 

 the strata on the two sides of a fault 

 are not parallel, the change of inclina- 

 tion being effected at the time of the 

 displacement, as shown in Fig. 201. 

 Upon the eroded surface of such dis- 

 located strata, by subsequent subsi- 

 dence, other strata may be unconformably deposited (Fig. 202). 



Law of Slip. — In faults the plane of fracture is sometimes vertical, 

 but much more generally it is more or less inclined. In such cases, in 

 by far the larger number of great faults, the strata on the upper side 

 (hanging wall) of the fracture have dropped down, while the strata on 

 the lower side (foot- wall) have gone up, as in Figs. 203 and 204 and 

 nearly all the previous figures. These are called normal faults. In 

 some cases of strongly-folded strata, however, the hanging wall seems to 

 have been pushed and made to slide upward over the foot-wall as if by 



Fig. 20^.— Unconformity on Faulted Strata. 



