Vol. VI] EVERMANN— DIRECTOR'S REPORT FOR 1916 257 



between the ocean and Petaluma and northwest in the vicinity of Freestone. 

 The type locaHty of the Merced, a Pliocene formation, is located in a clifif 

 section 3j/^ miles north of Mussel rock, where about 5000 feet of sands and 

 shales are exposed. These collections demonstrate the presence of Merced 

 strata resting unconformably upon an old erosion surface cut in ancient 

 Franciscan rock of probable Jurassic age. Sonoma tuff, a volcanic ash 

 and pumice deposit, was found interbedded with Merced strata in the 

 vicinity of Freestone and environs. This tuff occurs in very extensive de- 

 posits east of Napa, and occurs at the base of the Orindan formation near 

 Pinole. The Orindan formation which consists of lake deposits, landlaid 

 deposits, and interbedded volcanics is an extensive formation of the 

 Berkeley Hills. Probably it is the equivalent of the marine Merced. This 

 correlation is further reinforced by the discovery that the Lawlor ranch 

 beds of Sonoma Mountain, which consist of sands, gravels and clays of an 

 intervolcanic period rest upon andesite and Sonoma tuff. These beds have 

 yielded Neohipparion gidleyi Merriam. Neo-Hipparion also occurs in the 

 Orindan strata, and the Etchegoin of the western border of the lower San 

 Joaquin Valley. 



Three collections from an ancient lake deposit east of Petaluma were 

 obtained. Apparently a portion of the Petaluma lake beds represents a 

 period of time somewhat younger than any of the marine Miocene of San 

 Pablo age, as brackish water fossils were found in two localities. These 

 fossils are identical with forms which occur in a similar deposit in the 

 upper portion of the San Pablo formation of the Carquinez Straits vicinity, 

 a marine formation about 2000 feet thick. Here we have apparently a tran- 

 sition from marine to brackish water and finally to fresh water conditions in 

 this Miocene basin. 



In summary then : 



The region lying between Tomales Bay and the top of Sonoma Moun- 

 tain is a block which has been but little disturbed by folding or faulting 

 since the Pliocene. This wave-cut block has, however, been warped so 

 that it tilts from the sea towards Sonoma and Petaluma valleys, is up- 

 lifted near Freestone on the northwest to 1100 feet, but is only 500 feet in 

 elevation 20 miles south on the shore of Tomales Bay. 



The formations involved are in order, the Franciscan group, Petaluma 

 lake beds of upper Miocene age, Marine Merced Pliocene, Basalts, Ande- 

 sites, interbedded tuffs and sandstones of inter-volcanic period of the 

 Sonoma Mountain region, stream terraces and marine terraces of Pleisto- 

 cene age. 



Marine Miocene may exist beneath Sonoma Mountain, as is indicated by 

 oil sands and oil impregnated basalts at one locality east of Petaluma. 



Sonoma tuff is interbedded with the Merced formation at Freestone. 



The Merced formation is the correlative in a broad way of the Sonoma 

 tuff, the Pinole tuff, the Orindan, and probably the upper portion of the 

 Etchegoin-Jacalitos of the San Joaquin Valley. 



The Hayward fault on the west side of Sonoma Mountain exhibits many 

 characters which are due to recent movements which produced earthquakes 

 during recent historic time. 



