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CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



interviewed in Petaluma, that the horse remains, Neohip- 

 parion gidleyi Merriam, 18 were taken out of the beds asso- 

 ciated with the coal. The coal and freshwater chert indicate 

 that a considerable time would be required for their forma- 

 tion. 



These beds outcrop in a few other places southeast of 

 Lawlor's Ranch and, since they are less resistant than the 

 underlying tuff of the Sonoma group, are worn away easier 

 than the tuff, leaving a plateau underlain by tuff with the 

 Neohipparion beds in the scarp-like hillside. A generalized 

 N-S section running north from Adobe fort to the Peta- 

 luma Reservoir, shows the following sequence: 



Basalt 



Tuff 



Basalt 



Conglomerate 



Basalt 



Neohipparion beds 



Tuff 



Unconformity, base of Sonoma Group 



Petaluma formation 



From the end of this road to the top of Sonoma Mountain, 

 basalt predominates. These lavas on the Hayward Rift have 

 not been much disturbed and the vertical displacements may 

 be measured probably only in tens of feet. The top of So- 

 noma Mountain is to all appearances a plateau due to a 

 nearly horizontal sheet of the basaltic lava. The distribu- 

 tion of the lavas and tuffs is not uniform, since at some places 

 lavas rest directly upon the Petaluma formation while at 

 other localities tuff intervenes. Such is the case at the lo- 

 cality where bituminous sands occur in upper Lynch Creek, 

 about half a mile northwest of Mountain School, Santa Rosa 

 Quadrangle. At this place a fault whose exact position was 

 not determined has disturbed the Neohipparion gidleyi beds 

 and given them a pronounced dip of 25° W. These bi- 

 tuminous sands and carbonaceous shales rest upon basalt and 



18 Merriam, J. C, New species of the Hipparion Group from the Pacific Coast and 

 Great Basin Provinces of North America; Univ. Calif. Publ., Bull. Dep't. Geol., Vol. 

 9, No. 1, pp. 1-8, 1915. 



