SCIENTIFIC BACKGROUND 117 



rather than throughout long distances along the rift 



line. 



The actual fault displacements, either in verti- 

 cal, horizontal, diagonal or overthrust directions, may 

 vary from a few inches to a few feet in amplitude. 



When more or less vertical, the major rifts are sup- 

 posed to be deep fractures or breaks in the outer zone 

 (zone of fracture) of the earth's rock girdle, which 

 lose their identity as they approach the zone of flow- 

 age. The depths of the fractures may extend to twenty 

 miles or more — no one knows — until they are lost in 

 the more plastic zone of flowage. 



The amount of the nearly vertical displacements 

 or a group of closely spaced displacements, may vary 

 from a few inches, as may sometimes be seen in a fresh 

 roadside excavation, to as much as 15,000 feet, as is 

 evident in places along the north side of the San Gab- 

 riel Range; of over 10,000 feet along the east end of 

 the south side of the same highlands ; of over 10,000 

 feet between the summit of the San Bernardino High- 

 land and the San Jacinto Valley near Eden Hot Springs ; 

 and of at least two miles on the east side of the San 

 Jacinto Range, as may be seen at a glance by looking 

 upward to the top of San Jacinto Peak from the flat 

 of the Salton Sea. 



Let not the casual reader be appalled by the 

 totality of displacements or think that they represent 

 single catastrophic movements. They were made by 

 many small movements during long past epochs of geo- 

 logic time. All the great, geologic events have required 

 incomprehensible numbers of years for their accom- 

 plishment such as are hardly appreciable to the human 

 mind. The aggregate of these great, scarp-producing 



