GEOLOGIC HISTORY 219 



land (Noble and Kew), which prove the Pleistocene 

 ages of some of these rifts. The first Northwest group 

 of rifts of the submerged portion of California are 

 probably post-Miocene because in Miocene time the 

 islands were connected with the land, and the develop- 

 ment of the faults in later times is the best explana- 

 tion for the separation. 



AGES OF THE NORTH-SOUTH GROUPS OF 

 STRUCTURES 



It has just been shown that the North-South groups 

 of structures of the Great Basin Region were initiated 

 as far back as the early Miocene or Eocene epochs. It is 

 probable that the great rift or rifts which more or 

 less theoretically mark the east side of the Peninsula 

 Highland and the trends of the northern end of the 

 Colorado Depression are of similar age. Furthermore, 

 the structures of the two regions may have been at 

 one time more or less continuous before the great fault 

 line of the East-West and Northwest systems crossed 

 their paths. 

 RELATIVE AGE OF THE NORTHEAST TRENDS 



The northeast-trending faults of the San Gabriel 

 and San Jacinto highland regions are apparently cut 

 directly across by the east-west extending faults of 

 the Transverse Belt and by the north and west strik- 

 ing faults of the San Andreas-Puente Belt. They may 

 be of Pliocene or at latest, early Pleistocene age. This 

 assertion is a tentative one, however, owing to the 

 indefiniteness and incompleteness of the data which 

 now is possessed concerning these northeasterly ex- 

 tending structures. Movement has certainly taken 

 place along the Garlock fault since Pliocene time, as is 

 explained upon another page. 



