16 SOUTHERN CALIFORNIA GEOLOGY 



There is no way of ascertaining the entire length of 

 the San Andreas Rift. Its total extent may reach 

 thousands of miles instead of hundreds as is ordinarily 

 represented. Further details of this rift are given in 

 a later chapter.^ 



This southeastern portion of its course may for con- 

 venience be again subdivided into the Valyermo, Beau- 

 mont and Desert Sections. The first mentioned em- 

 braces all that part northeast of Cajon Pass. The 

 Beaumont section includes that portion adjacent to 

 the southwest side of the San Bernardino Plateau, 

 while the Desert section is southeastward from the 

 Beaumont section. The course of the San Andreas Rift 

 passes through the centers of population of Northern 

 California. But the Southern California portion of its 

 course lies far to the north and east of Los Angeles 

 and, for a great part, through an unpopulated coun- 

 try. The nearest point of approach of the mostly quiet 

 Valyermo section to Los Angeles is at least thirty 

 miles, and of the more active San Jacinto section some 

 sixty miles. 



The portion of this greater line which lies in the 

 bounds of the State of California is known as the San 

 Andreas Rift, and is considered the line of greatest 

 seismicity within the state — a statement which must 

 be modified as will now be explained. 



Personally, from long observation and familiarity 

 with the facts, I know that the line of maximum 

 seismicity does not altogether follow the lines above 



'Several detailed pai»€rs and monographs have been written on the San 

 Andreas rift — more than has been written on all of the other great fault 

 lines of California put together. Among these may be mentioned the great 

 monographic work of the Califoraia Earthquake Commission. Notably excellent 

 detailed studies and publications have been made in recent years by Mr. L. V. 

 Noble of Valyermo. 



