Vol. XI] D1CKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 543 



N. 60° W. The fossils belong to the genera Sphccrium, Pisi- 

 dium, Planorbis, Lymncna and Physa, and form an assembly 

 of species very similar to that found in the Tulare formation 

 of the Coalinga field, but the forms are all specifically distinct. 

 They are likewise separable from similar species in the Orincla 

 lake beds of the Berkeley Hills. It is believed wild ducks 

 distribute the eggs of the species belonging to these genera 

 at the present time, and as such means of distribution probably 

 existed during the Miocene time, it would seem that such 

 forms should prove useful in correlation with other upper 

 Miocene lacustrine beds. 



The Petaluma formation in the field northeast of Petaluma 

 is thrown into a series of narrow anticlines and synclines 

 which are not very persistent. Anticlines which are clearly 

 seen in one creek canyon will not be present in the next 

 canyon a half mile east of the first one. These shallow fold- 

 ings do not extend to Tolay Valley. In the hills west of 

 Eureka School, a well developed syncline was recognized and 

 its corresponding anticline parallels the northwesterly flowing 

 stream which heads at the Eureka School. As was stated 

 above, the Tolay fault limits this formation in a westerly 

 direction. Here the various members of the Petaluma for- 

 mation are in contact with the Franciscan group and along the 

 fault these lake beds are displaced greatly, steeply dipping 

 beds being the rule. The plane of this fault is almost verti- 

 cal, in one place measuring 75° to 80°. This fault is an old 

 one as the rocks of the Sonoma group cross it indifferently 

 in its southeastern extension. However, it may have been an 

 important line of movement during late Miocene or early 

 Pliocene. 



Merced Group, Pliocene 



This group was recognized in Santa Rosa Valley early in 

 the work of the Geological Survey of California, and Gabb, 

 in the volume on the Paleontology of California, described 

 several species of fossils collected from exposures on Mark- 

 West Creek. The thickness of these beds is not great nor are 

 they greatly folded in most places in the region of Mark West 

 Creek and Freestone. Osmont represents these beds as hav- 

 ing a very decided dip toward the center of Santa Rosa 

 Valley, but observations made at Wilson's Ranch show that 



