232 STRUCTURAL GEOLOGY. 



there were no erosion. But both the slipping and the 

 erosion have probably been going on slowly all the time, 

 and whether there be a cliff or not, depends on the age 

 of the fracture and the relative rate of slipping and ero- 

 sion. In many of the faults of the plateau and basin 

 region, the cliff still exists (though not as great as the dis- 

 placement), because of the comparative recency of the 

 fractures and dryness of the climate. The great Sierra 

 fault is marked by a steep slope of 8,000 to 10,000 feet 

 to the east, that of the Wahsatch of 8,000 feet to the 

 west. But in the Uintah fault, and in all the faults of 

 the Appalachian region, there is actually no surface-sign 

 of the fault (Fig. 136). We may stand astride of the 

 fissure. 



Two Kinds of Faults. Faults are of two kinds, ac- 

 cording to the direction of the slip. Fissures are nearly 

 always inclined, and therefore have what miners call a 

 foot (lower) wall and a hanging (upper) wall. More com- 

 monly the hanging wall drops down. These are normal 

 faults. But sometimes the hanging wall is pushed up 

 over the 'foot wall. These are called reverse faults. In 

 normal faults the broken parts are readjusted by gravity 

 (settling) ; in reverse faults the broken parts are crushed 

 together and forced to slide. The upper wall over the 



FIG. 138. Section across Yarrow Colliery, showing the law of faults. (After 

 De la Beche.) 



lower (Fig. 130) is :i reverse fault. Fig. 130 shows only 

 normal faults. 



Law of Slip. In cases of displacement of strata it 

 becomes often a matter of great importance, not only to 



