268 



PHYSICAL GEOLOGY 



great extent produced by faulting which caused the streams to flow 

 in fault valleys. 



Minor Features of a Fault Fracture. — When a fault fracture is 

 visible, it is often found that it is represented by a zone of angular 



rock fragments, often 

 cemented together to 

 form a fault or crush 

 breccia (Fig. 357), 

 sometimes several 

 yards wide. Some 

 important gold and 

 silver deposits occur 

 in the filling of such 

 breccia (p. 371). 

 When the breccia is 

 very resistant, as is 

 the case when the 

 filling is quartz, it 

 may stand in relief 

 after the surrounding 

 strata are denuded, 

 resembling a dike. 

 The side of a fault surface is often polished and striated by the move- 

 ment of the walls, so that it is possible to tell the direction of the 

 movement from the striations. Such a surface is called a slickenside 

 (Fig. 267). It resembles a glaciated surface but is usually more glazed. 

 Detection of Faults. — Topography, as has been seen, is not al- 

 ways a safe guide for the detection of faults, since their presence is 

 not always indicated by cliffs. 

 The most satisfactory evi- 

 dence of a fault is to be ob- 

 tained when a geological map 

 of a region is made, and it is 

 found that all of the neigh- 

 boring formations end upon a 

 more or less straight line. A sudden change in the soil which is made 

 apparent in the degree of fertility of neighboring fields, and the pres- 

 ence of rapids in streams which cross a fault are also indicative of the 

 presence of a dislocation. Springs often occur along fault lines, and 

 when a number of them are aligned they indicate, but do not prove 



Fig. 267. — A slickenside surface formed by lateral 

 movement along a fault plane. The rock of one side of 

 the fault was removed. (U. S. Geol. Surv.) 



Fig. 268. — Diagram showing "drag dip" near 

 a fault. (Modified after W. N. Rice.) 



