1915] Clark: Occurrence of Oligocene in the Contra Costa Hills 



21 



and when a larger fauna is obtained from the type section of the San 

 Lorenzo it is not at all improbable that these beds may be found to be 

 equivalent, in part at least, to the Agasoma gravidum beds of Contra 

 Costa County. 



SUMMARY 



1. In Contra Costa County, California, beds containing the fauna 

 of the Agasoma gravidum zone are unconformably below those con- 

 taining the fauna of the Area montereyana zone. This unconformity 

 is believed to be more than local and represents the line of division 

 between Miocene and Oligocene. 



2. In California there is apparently a very marked difference be- 

 tween the Agasoma gravidum fauna and that of the middle and lower 

 Miocene. The fauna! break between the Oligocene and Miocene in 

 Oregon and Washington appears to be as great as in California. 



3. The fauna of the Agasoma gravidum zone of Contra Costa 

 County may possibly belong to a higher horizon in the Oligocene than 

 the typical San Lorenzo. The evidence for this statement is chiefly 

 negative; future work may show a closer relationship to the San 

 Lorenzo. 



Transmitted April 23, 1915. 



