200 



PHYSIOGRAPHY 



slumping on the slopes of deltas and on the outer faces of the 

 continental shelves produces similar results. 



Many great earthquakes are connected with other forms of 

 crustal movement. As already noted, fissures are sometimes opened 

 in the surface of the land during an earthquake. This is best 

 seen where there is little or no soil, and where the solid rock lies 

 close to the surface. There is a great crack of this sort in Arizona 

 (Fig. 198), and similar fissures have been formed in New Zea- 

 land, Japan, and elsewhere during earthquakes. It is not always 

 clear whether the fissure should be looked on as the cause or the 



Fig. 198. Fissure produced by earthquake. Arizona. 



result of the earthquake. In some cases (p. 192) it is found that 

 one side of such a fissure is higher than the other, indicating that 

 the rock on one side was raised or that of the other sunk, or both ; 

 in other words, that the strata have been faulted. Faulting is 

 probably the cause of the great earthquakes. The slipping of one 

 great body of rock past another would cause vibrations which 

 would spread far from the center of disturbance. 



It is probable that most earthquakes are to be looked upon as 

 but one expression of the widespread movements to which the 

 crust of the earth is subject, movements which are due primarily 

 to the continued adjustment of the outside of the earth to a shrink- 

 ing interior. In general, these movements are too slow to produce 



