PHYSICAL CHANGES OF THE OUTER SHELL 75 



to overthrow completely any loose objects upon the table. When 

 an earthquake disturbs the sea bottom a series of waves is set in 

 motion, as in a pan of water sharply tapped on the side. These 

 earthquake waves (often wrongly called "tidal" waves) rush upon 

 the shore and, as in the Sicilian earthquake of 1908, may wash 

 away houses with all their occupants, and may even dash large ships 

 high upon the beach. 



Slow growth of faults. Almost all large faults, whether 

 of the normal or the reversed type, have probably grown 

 through a series of small slips separated by years or centuries 

 in which no movement occurred. Along the great fault at 

 the east base of the Sierra Nevada in California (Fig. 457), 

 the vertical displacement now amounts to several miles ; but 

 the last important movement along this fracture occurred as 

 long ago as 1872, and at that time the dislocation was in- 

 creased by only 25 feet. If so great a fault were to be made 

 all at once, the shock would probably wreck every building 

 within hundreds of miles. Most faults grow so slowly that 

 the scarp (cliff) on the upthrown side meanwhile suffers much 

 from the work of weather and running water and comes to be 

 furrowed by many ravines. 



UNCONFORMITIES 



In many places the older beds of rock are folded or faulted 

 or are cut by intrusive bodies, whereas the younger beds are 

 undisturbed. Thus in Figure 59 the upper layers do not con- 

 form in structure to those beneath. The lower strata were 

 doubtless nearly level when first deposited. If so, they have 

 since been tilted in connection with folding movements ; but 

 the tops of the folds were worn off before the sediments which 

 formed the upper strata were laid down. The two sets of 

 layers are therefore said to be unconformable and the contact 

 is an unconformity. 



Not all unconformities show such a discordance of bedding. 

 A bed of sandstone deposited on the surface of a planed-down 

 mass of granite shows just as clearly that the surface was 



