1914] DicJcerson : Eocene of the Santa Ana Mountains 269 



Partial List op Species from Turritella Ocoyana Zone — {Continued) 



2345 2322 2341 2329 2338 



Spisula, ef. catilliformis Conrad x 



Saxidomus, sp x x 



Tellina, cf. ocoyana Conrad x 



Venus pertenuis Gabb x 



Pelecypod, n. sp x 



Conus hayesi Arnold x 



Calyptraea costellata Conrad x ... x 



Fissurella, n. sp x 



Trophosycon, cf. kernianum (Cooper) x 



Thais vaquerosensis (Arnold) x 



Turritella ocoyana Conrad x x .... x 



Balanus, sp x 



Carcharodon tooth X 



GEOLOGIC HISTORY 



The earliest geological event in the region of the Santa Ana Moun- 

 tains of which we have certain knowledge was the deposition in early 

 Mesozoic time of limestones and other sedimentary rocks. According 

 to Mendenhall, in the Jurassic or early Cretaceous these beds were 

 subjected to intense deformation and metamorphism. After a period 

 of erosion probably covering at least most of Knoxville and Horse- 

 town time the region was depressed below sea-level and sediments of 

 Chico age were deposited, which were very coarse at the base and later 

 predominantly sandy, with an abundance of marine fossils. Deform- 

 ative movements at about the end of the Cretaceous resulted in the 

 cutting away of part of the Chico. Subsidence again occurred and 

 allowed the deposition of strata containing a Martinez fauna, after 

 which further deformative movements caused these rocks to be very 

 largely swept away, so that only limited masses remain. Sub- 

 mergence in Eocene time allowed the deposition of the upper Eocene 

 or Tejon, but these rocks were likewise almost entirely removed. In 

 lower Miocene time subsidence again occurred to the extent of several 

 thousand feet and permitted the accumulation, perhaps partly on land, 

 of some thousands of feet of beds. Uplift and deformation since lower 

 Miocene time brought about degradation of the region. Apparently 

 diastrophism ceased for a sufficiently long period to allow the Santa 

 Ana region to become one of low relief. This low-relief surface was 

 possibly continuous with the peneplain surface of the Perris Plain to 

 the east, described by Mendenhall. The bedrock portion of the Santa 

 Ana Range probably rose above this surface because of the greater 



