232 



University of California Publications. 



[ Geology 



palaeontologic and geologic studies in both regions may ulti- 

 mately give us a much more satisfactory statement of the rela- 

 tions than is now possible. 



Of the formations in which mammalian fossils have been 

 found west of the Sierra Range region there are none in which a 

 sufficient representation is known to offer more than a mere 

 suggestion as to time relationship to the Thousand Creek Beds. 

 Four formations in California — the Pinole Tuff and Orindan 

 freshwater formations of the San Francisco Bay region, and the 

 largely marine Jacalitos and Etchegoin formations of the western 

 San Joaquin Valley — contain fragmentary mammalian remains 

 which suggest a late Miocene to Pliocene stage. 



The Pinole Tuff, overlying the San Pablo formation at San 

 Pablo Bay, contains a few fragmentary mammalian fossils, among 

 which is a horse of an advanced protohippine type. The type 

 of horse present here might represent late Miocene or early Plio- 

 cene. It is of a stage at least as advanced as that of the Plio- 

 hippus species found at Thousand Creek. 



In the Orindan and Siestan formations which overlie the 

 Pinole Tuff a few remains have been found at rather widely 

 separated localities. They include a large mastodon, a species 

 of Neohippariou near N. richthofeni, a small camel, a peccary, 

 and the type specimen of Eucastor lecontei. This fauna would 

 seem to represent a very late Miocene or early Pliocene stage, but 

 occurs above the Pinole Tuff with PlioMppus'. 



In the Jacalitos formation of the western San Joaquin Val- 

 ley, as also in the Etchegoin formation above it, scattered horse 

 teeth have been found representing a species of Plioliippus about 

 as advanced as that of the Pinole Tuff. Taken by itself, this 

 form would be considered as representing late Miocene to Plio- 

 cene time. 



The stages of evolution represented by the protohippine forms 

 of the Pinole Tuff and the Etchegoin formation are approxi- 

 mately the same, so far as can be determined with the very 

 fragmentary material at hand. They are also at approximately 

 the stage of advance shown in the Thousand Creek species. This 

 may, however, not be taken as suggesting more than that these 

 California formations are to be included in a period covered 



