540 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



quarter mile north 20° W. of Mountain School in the Santa 

 Rosa Quadrangle. The sandstone in places is thoroughly 

 impregnated with oil and droplets of oil were found in the 

 vesicular cavities of the overlying basalt. There are no other 

 indications of the Monterey group beneath Sonoma Mountain 

 that were found but it seems very probable that the basalts 

 and tuft's of the Sonoma group are underlain by the shales of 

 the Monterey group. The clays and sandstones of the Peta- 

 luma formation probably intervene in many localities. The 

 Monterey group may underlie the Petaluma formation in the 

 region southeast of the Mountain School. What would pre- 

 serve this group from erosion here while it was entirely re- 

 moved from the whole area only 4 miles west across Peta- 

 luma Valley? As has been pointed out above, the Tolay fault 

 has not been active since late Miocene or early Pliocene but 

 during this time the eastern or downthrown block may have 

 been so lowered beneath the general base level of that time 

 that it was in this manner preserved from the general destruc- 

 tion which the western block suffered. Whether this area 

 around the Mountain School has any economic oil possibilities 

 is problematic, although the writer is not prepared to con- 

 demn the area absolutely, yet the uncertainty of a source of 

 oil is very liable to make this region a pecarious one for 

 experimenting. 



Petaluma Formation, Upper Miocene 



This formation is confined to the northeast corner of the 

 Petaluma Quadrangle and the adjoining southeast corner of 

 the Santa Rosa Quadrangle. Stratigraphically, it is uncon- 

 formably overlain by the Sonoma group of volcanics and 

 their associated members on its eastern boundary while its 

 western limit in the Petaluma Quadrangle is marked by the 

 Tolay fault which separates it from the Franciscan in this 

 vicinity. On the northwest, the incoherent sands of the Mer- 

 ced formation mantle it in the vicinity of Penn Grove. Litho- 

 logically this formation is characterized by the great abun- 

 dance of clays, but only in certain stream canyons does one 

 obtain opportunity to observe them. Elsewhere they have 

 readily weathered into a thick, heavy soil. The different 

 lithologic facies are typically exposed along the Lakeville- 

 Sonoma road between Lakeville School and Eureka School 



