384 University of California Publications in Geology [Vol. 7 



leaves of trees. Professor W. L. Jepson, who has examined 

 these specimens, considers the leaves as representing Acer, sp., 

 Sterculia or Acer?, and Dirca, sp. 



AGE OF THE ORINDAN AND SIESTAN VERTEBRATE FAUNAS 



Although the vertebrate remains from the Orindan and 

 Siestan are probably obtained from horizons comprising quite 

 a wide range in time, they represent collectively the greater part 

 of the known land fauna of this region in a period following 

 the San Pablo Miocene and preceding the Pleistocene. Such 

 field work as has been done up to this time does not lead us to 

 expect large collections from these formations, though the dis- 

 covery of strata rich in determinable remains is always possible. 

 The Siestan beds seem to offer a possible fruitful field for future 

 collecting. 



The mammalian remains known from both the Orindan and 

 Siestan up to the present time all represent forms such as might 

 be expected in the late Miocene or in the earliest Pliocene, but it 

 will be necessary both to have better material from the Orindan 

 and Siestan and to have well-known faunas of western Miocene 

 and Pliocene for comparison before the last word on the age 

 determination can be pronounced. 



The late Tertiary mammalian faunas which are most avail- 

 able for comparison with the forms from California are those 

 of the Basin region, including the Tertiary beds of the Mohave 

 Desert region of California, those of the Cedar Mountain region 

 of Nevada, and the Thousand Creek beds of northern Nevada. 

 All of these faunas are as yet imperfectly known, but there 

 seems reason for considering the Thousand Creek as representing 

 early Pliocene, and the deposits of the Mohave and Cedar Moun- 

 tain regions as at least in part near late Miocene. 



Compared with these faunas there seems in some respects less 

 suggestion of similarity with Thousand Creek than with the other 

 two. On the other hand, there is no indication of identity with 

 the so-called Mohave fauna of the Barstow region. The beds 

 referred to the Rosamond series, in the Ricardo region on the 

 western border of the Mohave Desert, contain a fauna with 



