72 



University of California Publications. [Geology 



Prom the above statement, and his petrographical descrip- 

 tions, it appears that his asperites correspond to the St. Helena 

 rhyolite, and his older andesites to the Mark West andesite of 

 the present writer, while his andesitic tuff is the Sonoma Tuff, 

 which is extensively developed just southwest of the Oat Hill 

 Quicksilver Mine. 



AGE OF THE ABOVE SERIES. 



In the excellent section near Freestone the Sonoma Tuff 5 , 

 containing' marine shells, is resting horizontally, and strictly con- 

 formably, upon marine sandstones and conglomerates non-vol- 

 canic in their nature and containing shells referred doubtfully 

 by Gabb to the late Miocene. On the eastern side of Santa Rosa 

 Valley, however, near Mark West Creek, some 700 feet of Mark 

 West andesite occurs, directly underneath and conformable with 

 the tuff, while to the south, on the head-waters of Petaluma Creek, 

 this andesite may be seen resting unconformably across the eroded 

 edges of nonvolcanic sandstones and conglomerates very similar 

 in appearance to that near Freestone, but of fresh-water origin 

 and containing a typical upper San Pablo fossil, Cyrena Cali- 

 f arnica. In these latter strata also, at Lawlor's Ranch, horse- 

 - teeth very similar to those of the Orindan have been found, 

 which Dr. Merriam refers tentatively to late Miocene. The Mark 

 West andesite appears therefore to certainly be post-San Pablo, 

 and probably post-Orindan in age. 



At Rodeo, on San Pablo Bay, occurs an exposure of pumi- 

 ceous tuff indistinguishable in appearance from that described 

 above in the writer's territory. It rests conformably upon the 

 typical San Pablo, but recently Dr. J. C. Merriam found horse- 

 teeth and other vertebrate remains in it, showing it to be not 

 older than Pliocene. Dr. J. W. Giclley kindly examined these 

 horse-teeth, and reports as follows : 



' ' Number 2142 is an upper molar of Protohippus. ' ' 



On the west side of Santa Rosa Valley, along the Russian 

 River, the Sonoma Tuff lies conformably beneath the Wilson 

 Ranch Beds, which are certainly of Merced age, probably about 

 the horizon represented by the Capitola section and the base of 

 Seven Mile Beach section. The Sonoma Tuff is, therefore, pretty 

 certainly of lower Merced age. 



