582 PROCEEDINGS OE THE SAINT LOUIS MEETING 



undoubted Miocene sediments two divisions have been more or less perfectly de- 

 scribed by former writers, though our present knowledge of them is far from com- 

 plete. They have been called the Monterey shales and the Pescadero sandstones. 

 The Pescadero sandstones, supposed to be the lowest member of the Miocene 

 series, was first described from its occurrence along the coast north of Santa Cruz. 



Locally its aggregate thickness has been estimated at over 11.000 feet. In the 

 Carrisa valley of San Luis Obispo county a similar series 14,000 feet in thickness 

 Occurs, dipping uniformly to the northeast at an angle of from 40 to 50 degrees. 

 The series consists of alternate beds of shales and sandstones, of which the latter 

 forms the predominating element. There are three distinct divisions composed 

 entirely of the shales. The sandstones are usually more fossiliferous and consti- 

 tute four-fifths of the series. 



The Monterey shales constitute the most characteristic portion of the Miocene 

 rocks. At least four elements enter generally into their composition, and locally 

 are more or less segregated. These elements are organic silica, volcanic ash, sand 

 and lime carbonate. There are three stratigraphic divisions that may be distin- 

 guished in most districts, though not of equal importance. A representative sec- 

 tion of the Miocene shales exposed in western Santa Barbara county comprises the 



following : 



Feet. 



Diatom shale 800 



Hard s-ilicious shale.... 2,400 



Sandy beds with lime , 200 



The sandy beds at the base of the series contain fossil invertebrates, among 

 w 7 hich the characteristic forms are Amuseum, Pecten crassicardo Pecten discus Conrad, 

 Scutella brewerianus? Gabb., Leda sp. cardium, Terebratella {Laqueus), etcetera. In 

 some districts, as in the vicinity of Temblor, Kern county, there is more shale and 

 sandstone below the beds characterized by these fossils. 



Late sedimentary Beds 



Two or more series of late sedimentary rocks are often found overlying the Mio- 

 cene beds, the more widely distributed being the San Pablo beds. The San Pablo 

 rocks are mostly soft sandstones, often very fossiliferous. In Fresno county the 

 thickness is locally more than 6,000 feet. The Merced series, or almost its equiva- 

 lent, occurs in western Santa Barbara county and in Ventura and Los Angeles 

 counties. Fresh-water deposits are widely spread through all the larger basins, as 

 in the San Joaquin and Salinas valleys, and apparently in the desert region of 

 Mojave. Glacier deposits in the form of drift occur abundantly about the higher 

 rangers and on the borders of the Kern valley, having in some places a depth of 

 200 to 300 feet. 



Origin of Bitumen 



In all of the important petroleum districts of this region organic remains of mi- 

 croscopic size are found in sufficient numbers to support the view of an organic 

 origin for the bitumens. Diatoms, foraminifera, and radiolaria occur in enormous 

 numbers in some of the strata, and very often in strata saturated with petroleum. 

 Sufficient notice has not yet been taken of this fact. 



Remarks were made by A. C. Lawson. 



