SCIENTIFIC BACKGROUND 153 



of the northern end lies in the United States. On the 

 desert side the fault lines of the belt extend southeast- 

 ward to and across the Mexican line. Its greatest 

 length in California is over two hundred seventy-five 

 miles and it extends an unknown distance into Mexico 

 —probably to and beyond the Isthmus of Tehuantepec. 

 On the Pacific side the master faults of this belt appar- 

 ently extend southeastward for 150 miles in Southern 

 California, and for an indefinite distance into Mexico. 

 The width of this belt is not limited to California but 

 it may extend still farther east across Southern Ari- 

 zona, New Mexico and into Trans-Pecos Texas and for 

 an equal distance across Northern Mexico. 



Some members of the belt on the Desert side ap- 

 parently terminate abruptly against the Oarlock fault 

 at the northwest. Others like the Mill Creek and San 

 Jacinto rifts branch off southward from the San An- 

 dreas rift, while still others appear to be southward 

 continuations from the flexed structures of the Ven- 

 tura Ranges. 



Ever>"vvhere the faults show a similarity of char- 

 acter, each making a strong lineament of the physiog- 

 i-aphy — fault scarps, often old and worn, mountain and 

 valley borders, or linear grooves and valleys in the 

 face of the landscape. 



The group includes at least ten master fault lines 

 of unusual length and topographic conspicuity and ap- 

 parently of profound and mostly normal vertical dis- 

 placements. Beginning at the east and proceeding 

 southwestward these may be enumerated as follows: 

 Cadiz, Newberry, Old Woman, Mill Creek, San An- 

 dreas, San Jacinto, Reche Canyon, Grapevine, Elsinore, 

 San Onofre and Inglewood faults. 



