168 SOUTHERN CALIFORNIA GEOLOGY 



the structure of this rift hne, although I have closely 

 reconnoitered it. 



In southern San Diego County there are two, if not 

 three, north-south extending faults which terminate 

 to the northward against the line of the Elsinore Rift. 

 One of these passes near Campo and another near 

 Jacuba. Their southward extent crosses the border 

 into Mexico. 



THE LAKEWOOD RIFTS OF THE FERRIS 

 PENEPLAIN 



Many subordinate interior fault lines lie between the 

 Elsinore and San Jacinto Faults and are parallel to 

 them. These occur in alternations of long ribbon-like 

 strips of level plains and mountains some twenty-five 

 miles wide, which extend southeastward through the 

 northern part of the Peninsula Highland region, or the 

 "Perris Peneplain," lying between the San Jacinto and 

 the Santa Ana Mountain blocks of higher altitude on 

 either side of it. 



The portion of the Ferris Peneplain above mentioned 

 is not a true peneplain of base leveling erosion, but is 

 composed of several ranges and valleys which are alter- 

 nately up-and-down thrown blocks between parallel 

 fault lines of the Northwest System. Some of the 

 monadnock-like, granite hills, of which the Lakewood 

 Mountains are the most conspicuous members, rise one 

 thousand feet above the surrounding level of the San 

 Jacinto Valley, towards a common level of two thou- 

 sand six hundred feet. The elongated belts of hills are 

 also checkerboarded by minor cross rifts. 



The writer at present entertains the tentative hy- 

 pothesis that the summits of the hills of the Ferris 

 Peneplain belt are the residuals of a former horst-like 



