404 University of California Publications in Geology [Vol. 9 



100 feet of green-gray sandstone and shale with limestone concretions 



marking the upper limit of the Eocene. 

 300 feet of green-gray, glauconitic shale. 

 200 feet of massive, thin-bedded, buff sandstone. 



100 feet of impure, gray limestone with thin strata of hard gray, medium- 

 grained sandstone. 



600 feet of massive, medium-grained, yellow, non-fossiliferous sand- 

 stone in contact with the andesitic core. 



Resting upon the Eocene are 500-600 feet of lone gravels and sands. 

 The two uppermost Eocene members, which are very fossiliferous, can be 

 easily recognized in the field by the bright red, clay soil formed through 

 their decay. The limestone concretions which are sometimes yellow, 

 contain many small, dark green to black, rounded grains. When these 

 are carefully examined with a hand lens they are found to be foraminiferal 

 casts composed of glauconite. The green shales as well as the sandstone 

 also contain glauconite and foraminifers. The strata are lithologically 

 similar to certain horizons in the Martinez, but the character of the fauna 

 here compels us to abandon the notion that green glauconitic sandstone 

 and shale are absolutely indicative of the Martinez in the middle Cali- 

 fornia region. Glauconitic sandstone also occurs in the Tejon of the 

 Mount Diablo region. 



On the east side of the South Butte the writer has mapped another 

 Eocene area which has about the same sequence as the above. Thin strata 

 of coal are reported by Watts and later by Lindgren and Turner, from the 

 lower portion of this section. 



According to H. Hannibal some of the area south of South Butte 

 mapped as lone is Cretaceous. 



In the section given above the ' ' 100 feet of impure gray lime- 

 stone Avith thin strata of hard gray, medium-grained sandstone" is 

 probably the upper member of the Chieo Cretaceous, as fossils 

 Spisula ashburncri (Gabb), Trigonia evansana Meek and Gyrodes 

 expansa Gabb were found in a stratum of sandstone at its base. The 

 "600 feet, of massive . . . sandstone" is also Chieo Cretaceous. 



Description of the Sutter Formation 



Resting with marked unconformity upon the Chieo and the 

 Tejon are 500-600 feet of strata which are composed chiefly of 

 rhyolitic ash, rhyolitic tuff-breccia, thin flows of rhyolite and con- 

 glomerate containing rhyolitic and quartz pebbles. This formation 

 is overlain by lava flows and mud flows which consist of andesitic 

 material. Evidence of a time-interval between these two volcanic 

 formations is given by a marked difference of dip between them 

 throughout the field. The materials in the Sutter formation were 

 derived chiefly from the rhyolitic volcanic rocks (mapped by 

 Lindgren as Nr), as the lava composing the thin flow noted above 



