STRUCTURAL GEOLOGY, 



517 



result of the friction of movement. Such surfaces are sUckensides 

 (Fig. 424). A sHckenside surface has some resemblance to a glaciated 

 surface, but generally gives evidence of greater rigidity between the 

 moving surfaces. 



Faults are of two general classes, normal and reversed. In the normal 

 fault (Fig. 418) the overhanging side is the downthrow side, i.e., the 

 downthrow is on the side towards which the fault-plane inclines, as 

 though the overhanging beds had shdden down the slope. Normal 

 faults, as a rule, indicate an extension of strata, this being necessary to 

 permit the dissevered blocks to settle downwards. In the reversed 



Fig. 425. — Perspective view and vertical section of a thrust-fault. 

 (Willis, U. S. Geol. Surv.) 



fault, 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 shown to be the case where an overfold passes 

 into a reversed fault (Fig. 420). Reversed faults are further illustrated 

 by Figs. 425, 426, and 427. Where the plane of the reversed fault 

 approaches horizontality, the fault is often called a thrust-fault, or an 



