THE STRUCTURE OF THE EARTH 



269 



the presence of a fault. Dikes of lava also sometimes occur in fault 

 fractures. In regions of horizontal rocks faults can often be traced 

 along the surface some distance from the fault line (p. 277) by the 

 upturned edges of the strata (drag dip), produced by the friction 

 along the fault surface of the edges of the strata during the faulting 

 (Fig. 268). 



Origin of Faults. — In seeking an explanation for normal faults 

 the fractures along which the movements occurred must first be 

 found. These are often joints which, as has been seen, have probably 

 been formed by tortional strains (p. 259). Normal faults indicate a 

 stretched condition of the crust, which permitted the unequal settling 

 of blocks bounded by joints or other fractures. In the formation of 

 grabens and horsts (Fig. 257, p. 263) there appears to have been first 

 a compression which 



arched the strata, 

 followed by a relief of 



the pressure, which nlS38^i?=| 



Fig. 269. — Diagram showing a fault shading into a fold. 

 (After De Martonne.) 



permitted the set- 

 tling of the blocks of 

 the arch. Reverse 

 faults are evidently 

 the result of lateral 

 pressure and are best developed in highly folded and distorted 

 rocks. Thrust faults often pass horizontally into folds, and verti- 

 cally they sometimes shade gradually into more or less gentle 

 folds (Fig. 269). The origin of the force which produced folding 

 will be taken up more fully under the discussion of the formation 

 of mountains. 



Rapidity of Fault Movements. — At irregular intervals dislocations 

 of from a fraction of an inch to 20 or more feet have taken place in a 

 few minutes along fault planes. In Owen's valley, California, in 

 1872, a slipping occurred along a line 40 miles long which resulted in a 

 throw of from 5 to 20 feet. Along a fault 50 miles in length a dis- 

 placement of as much as 30 feet occurred in Japan in 1891. Such 

 faulting always produces earthquakes (p. 281). In some mines 

 faulting is observed to be taking place continually, but at a slow rate. 

 The total displacement resulting from such movements, if long-cop- 

 tinued, will necessarily be great, but no surface features may mark its 

 presence, since erosion may cut away the upthrow side as fast as it rs> 

 formed. 



