Clark: The San Lorenzo Series of Middle California 111 



by the writer with beds generally recognized as Oligocene in Oregon, 

 Washington and Vancouver Island, B.C. The close relationship 

 of the beds in these northern localities with those in California is 

 shown by the fact that over forty per cent of the species obtained 

 from the Astoria series of Mount Diablo have also been found in these 

 northern Oligocene beds. 



The fauna of the Oligocene of Mount Diablo is apparently not so 

 old as the lowest Oligocene found in the San Emigdio Mountains at 

 the south end of the San Joaquin Valley, California ; it is very prob- 

 able, though the evidence is not as yet conclusive, that this Mount 

 Diablo Oligocene fauna belongs to a somewhat higher horizon than 

 does the lowest Oligocene fauna in Washington, Weaver's Molopo- 

 phorus lincolnensis zone, but below the horizon containing the typical 

 Aciia gettysburgensis fauna. 



This Oligocene fauna, taken as a whole, is very distinct from that 

 of the Lower Miocene. It is believed that this difference is largely 

 due to the time factor. This is borne out by the fact that in California 

 there is a large stratigraphic break which separates the Oligocene beds 

 from those of the Lower Miocene. 



