1 915 ] Clark: Occurrence of Oligocene in the Contra Costa Hills 13 



of a stratigraphic break between the Tejon and the Agasoma gravidum 

 beds. However, in almost every section a typical Tejon fauna was 

 found not very far below beds containing the Agasoma gravidum 

 fauna. The great difference between these two faunas leaves very 

 little doubt as to the presence of a large break. 



Relation to Area montereyana Zone. — The Agasoma gravidum beds, 

 just described, are overlain unconformably by beds containing an 

 almost entirely different fauna. These represent the base of the Area 

 montereyana zone, and contain the following species : 



Area montereyana Osmont Ostrea titan Conrad 



Marcia oregonensis Conrad Agasoma barkeriannm Anderson 



Mulinia densata Conrad Crepidula princeps Conrad 



Panope estrellana Conrad Fusus stanfordensis Arnold 

 Pecten, cf. bowersi Arnold 



A number of new species were found at this horizon. Of those 

 listed above, the only one which has been found in the Agasoma 

 gravidum beds is Panope estrellanus. 



The Area montereyana beds in this section have an estimated thick- 

 ness of about 800 feet. The basal 100 feet consists of coarse sandstone 

 in which cross-bedding is very common, together with lenses of con- 

 glomerate and shale. This is followed by about 100 feet of pearl-gray 

 shale, above which the beds consist for the most part of a fine gray 

 sandstone. 



The line of contact between the Area montereyana beds and the 

 Agasoma gravidum beds is marked by conglomerate and coarse sand- 

 stone. The conglomerate is lenticular and, in some localities, has a 

 thickness of from ten to fifteen feet ; in other localities it may be 

 almost entirely absent. The boulders and pebbles found in it Avere 

 derived from many sources ; both igneous and sedimentary rocks are 

 present. 



The evidence in this section for an unconformity between the 

 Area montereyana beds and the Agasoma gravidum beds is as follows: 

 ( 1 ) It is suggested by lithology ; conglomerates and coarse sandstone 

 occur along a line of contact, below which there is a sudden change 

 to a fine sandstone. (2) A contact with irregularities of considerable 

 size has been observed between these two horizons ; in certain local- 

 ities the Agasoma gravidum beds vary in thickness along the strike, 

 due apparently to erosion ; the Area montereyana beds have long been 

 known to rest locally upon the Eocene. (3) Fossiliferous boulders 



