64 



GEOLOGY 



place when the beds are horizontal, or when they dip. When they 

 are horizontal, the direction of the fault-movement is horizontal. 

 The fault-movement may also be horizontal when the plane of slip- 

 ping is vertical, or when it has any angle of hade. In this case, 

 the fault shows itself at the surface by the displacement of straight 

 lines, such as fences (Fig. 35). The fault at San Francisco in 1906 

 was of this type. 



Where there is vertical movement in faulting, it is not usually 

 possible to say whether the lower side has sunk, or the upper risen, 

 or both, though in normal faults, sinking is probably the dominant 



Fig. 37. Fig. 38. 



Fig. 37. A branching fault. (Powell, U. S. Geol. Surv.) 

 Fig. 38. Diagram of a fault passing into a monoclinal fold. 



movement. In any case, one side projects above the other, and 

 the cliff is a fault-scarp. In the course of time the scarp is destroyiM 1 

 or obscured by erosion, and most faults are so old that this has 

 taken place; but occasionally fault-scarps of mountainous heights 

 are found, as along the east face of the Sierras, and along some of 

 the basin ranges of Utah, Nevada, etc., but they are much modified 

 by erosion (Fig. 36) . Great faults are probably the sum of numerous 

 small slippings distributed through long intervals of time. Fault- 

 ing is probably the most common cause of earthquakes. 



Sometimes a fault branches, and sometimes the faulting is 

 distributed among a series of parallel planes at short distances 

 from one another, instead of being concentrated along a single 

 plane, thus giving rise to a distributive fault (Fig. 37). 



Faults are observed to die out gradually when traced horizon- 

 tally, sometimes by passing into monoclinal folds (Fig. 38), and 



