188 ARNOLD AND STRONG — CRYSTALLINE ROCKS OF SAN GABRIEL 



tions the probable formation of a great fault along the northern front of 

 the chain at about the opening of the Quaternary era. 



Age of the San Gabriel Mountains 



There has been great divergence of opinion regarding the age of the 

 San Gabriel mountains and their correlation with the other prominent 

 ranges of California. Trask considered the granitic rocks of the chain 

 as primitive, while Whitney placed their elevation as post-Cretaceous. 

 Fairbanks, than whom no one is better qualified to speak, has this to 

 say regarding the age of the plutonics and metamorphics of southern 

 California : * 



" I believe that the great convulsion which upheaved and metamorphosed the 

 older rocks and intruded granite into them took place as it did in central and 

 northern California between the Cretaceous and the Jurassic. There is no break 

 in the line of granites and crystalline schists the whole length of California." 



It appears quite probable, however, in the light of some recently ob- 

 tained evidence, that at least the greater part of the elevation of the San 

 Gabriel mountains took place during either the late Eocene or Oligocene 

 period. Tilted strata of sandstone and shale of lower Eocene age, found 

 at an elevation of over 5,000 feet in the vicinity of Rock creek, on the 

 northern face of the range, show that the chain has been elevated at 

 least 5,000 feet since the deposition of the lower Eocene.f It has also 

 recently been discovered that the conformable series of conglomerates, 

 sandstones, and shales which flank the San Rafael hills on the south 

 and underlie the southern portion of the city of Pasadena are of Mio- 

 cene age.J As the conglomerates of this formation (for which the name 

 Pasadena is here proposed) rest on and are composed of the San Gabriel 

 plutonics and metamorphics, it is evident that the chain is certainly pre- 

 Miocene, although quite a little elevation has taken place since the depo- 

 sition of this Miocene formation. 



General Character of the Rocks 



The following rocks have been found by the writers in the San Gabriel 

 mountains and are described in this paper : Biotite-granite, quartz -mon- 



* H. W. Fairbanks : Geology of San Diego county ; also a portion of Orange and San Bernardino 

 counties. Eleventh Ann. Report of the California State Mineralogist, 1903, p. 119. 



f Fossils from Rock creek recently sent by Dr W. C Mendenhall to Doctor Dall for determina- 

 tion proved to be of lower Eocene (probably Martinez) age. 



X The senior author and his father, Delos Arnold, have recently obtained a good series of fossils 

 from Raymond hill, Pasadena, which proves the age of this formation to be either lower or middle 

 Miocene. 



