SCIENTIFIC BACKGROUND 109 



as has been done in regions where the folding and 

 faulting are relatively older. Nevertheless, while the 

 physiographic features of Southern California do thus 

 reflect their structural origin, they also show variation 

 in the degree of this relationship. 



PREVIOUS STUDIES OF RIFTS 

 Some of the individual rifts of Southern California 

 have been previously studied by geologists, including 

 Fairbanks, Lawson, Noble, English, and others. For 

 nearly twenty years I have been making a more or 

 less close study of them in Southern California. A 

 manuscript map of these was made by me fifteen 

 years ago, but its publication has been delayed pend- 

 ing the constant accumulation of further data, and 

 other obstacles not necessary to mention. This map 

 shows material differences with the fault map later 

 made by the Seismologic Society of California. 



Excellent topographic maps and the relief model of 

 the hitherto but little known desert side of Southern 

 California made by the Colorado River Surveys of the 

 Los Angeles City Bureau of Power and Light present 

 the physiography in beautiful detail and show many 

 rift lines hitherto undescribed. Likewise they fur- 

 nish details for verifying previously formed concepts 

 of the wonderful phenomena of the region mentioned 

 which I had seen and studied in the field many years 

 ago, and thus they enable me to present my previous 

 conclusions with greater certainty and assurance. 



FAULT STRUCTURES AND EARTHQUAKES 



Most earthquakes, as I have noted earlier in this 

 paper, are vibrations set up in the earth's crust as the 

 result of slipping or movement along portions of fault 



