SCIENTIFIC BACKGROUND 119 



movements have been no more sudden than have the 

 many great invasions of the ocean waters back and 

 forth across most of the area of our continent. 



In instances of drifting the two sides of the fault 

 displacements have been sliding past each other in hori- 

 zontal directions instead of vertical, with stresses and 

 strains like those encountered by placing the flat sides 

 of two books tightly together, and dragging them in 

 opposite directions with jerky movements equivalent 

 to alternations of stresses and releases of strain. 



Horizontal movements of this kind in California are 

 supposed to have chiefly characterized the past history 

 of the San Andreas rift. One of these movements is 

 believed to have accompanied the San Francisco earth- 

 quake of 1906. Dislocation of the stream lines by such 

 movements is indicated by Noble who says that in 

 one place, three miles south of Cajon Pass, four ravines 

 which come down from the San Bernardino Mountain 

 slopes are displaced 150 feet to the northwest. Noble 

 also suggests a horizontal displacement of twenty-four 

 miles along the San Andreas fault. He does not con- 

 sider this a positive conclusion, however. 



Other suggestions of horizontal displacement along 

 this rift, besides those given by Noble in the paper 

 previously cited, are the dislocations in the stream pat- 

 terns of the Santa Ana River and Mill Creek as they 

 cross the rift zone on leaving the highland, and the 

 apparent drag to the southward of the east ends of the 

 supposed east-west rifts which mark the summit of 

 the plateau itself. It has even been suggested that the 

 entire southwest face of the San Bernardino plateau 

 has been moved some twenty miles from a former po- 



