24 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
block, with a very straight and abrupt fault-scarp facing the northeast and overlooking 
the Perris Plain. This is an elevated, and as yet little dissected, peneplain with rem- 
nants of Tertiary or later deposits resting upon it, indicating that it has, in part at least, 
but recently been resurrected from a buried condition. In San Diego County the Santa 
Ana Mountains find their prolongation in a less regular and broader group of ridges, but 
doubtless the same tilted block structure prevails to the international boundary and 
beyond, since the northeast scarp appears to persist in the same general trend, and the 
same type of consequent drainage characterizes the seaward slope. Still east of the line 
of the scarp in southern San Diego County, there is another orographic block, bounded 
on the east by a very recent and very precipitous scarp looking out over the desert.’ 
To the northwest the range becomes subdued in the Puente Hills, where a broad anti- 
clinal structure replaces in part the deformation by faulting. In two notable instances, 
and perhaps in others, the seaward streams of the Santa Ana Mountains cut entirely thru 
the range and drain the valley-land beyond its northeasterly scarp. These are the 
Santa Ana and the Santa Margarita Rivers. They are both probably antecedent to 
the more acute phases of the tilting of the region and have persisted in their course 
during the development of the fault-scarp. 
The San Jacinto Mountains form an important feature of the region as a bold ridge 
with northwest-southeast trend lying between Perris Plain and the northern end of the 
Colorado Desert. Both sides of the range are precipitous and are probably determined 
by faults. On the southwest side there were notable ruptures of the ground in the 
earthquake of 1898, indicating that the fault on that side is still in active development. 

1 Verbal communication from Dr. H. W. Fairbanks. 
