410 GEOLOGY 



basin ranges of Utah and Nevada. As a rule, they are much 

 modified by erosion (Fig. 331). 



The rock on either side of a fault-plane is often smoothed as 

 the result of the friction of movement. Such surfaces are slicken-. 

 sides. A slickenside surface has some resemblance to a glaciated 

 surface, but is generally more glazed. 



Faults involving vertical displacement along joints are of two 

 general classes, normal (or gravity) and reversed (or thrust). In 



Fig. 331. A fault-scarp, modified by erosion. The triangular faces 

 rising abruptly above the plain at the ends of the spurs are remnants 

 of the scarp. (Davis.) 



the normal fault (Fig. 330) the overhanging side is the downthrow 

 side, i. e., the downthrow is on the side towards which the fault- 

 plane declines, as though the overhanging beds had sliddon down 

 the slope. Normal faults, as a rule, indicate an extension of strata, 

 this being necessary to permit the dissevered blocks to settle down- 

 wards. In the reversed fault (Fig. 34), the overhanging beds 

 appear to have moved up the slope of the fault-plane, as though 

 the displacement took place under lateral pressure. This is clearly 



