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



around Petaluma this same relationship is seen in several 

 places. No attempt was made to separate this group in the 

 mapping, but nearly all its characteristic rocks occur within 

 this area. Since this terrain is merely an extension of the 

 Franciscan which Lawson 15 mapped in detail in the Tamalpais 

 Quadrangle, of the San Francisco Folio, the reader is referred 

 to this work for details. Chert, glaucophane schist, garnet 

 schist, actinolite schist, sandstone and igneous rocks associated 

 with this group, — serpentine and basalt — were recognized in 

 this region. A conglomeratic limestone was also noted a few 

 miles south of Petaluma. This limestone seemed to be a 

 beach deposit and although it appeared as if it might be fos- 

 siliferous, no samples were found which contain unmistakable 

 organic remains. This deposit will be described in detail 

 under Economic Notes. 



No attempt was made to work out the structure within the 

 Franciscan, but only the nature of the contacts between this 

 group and the Tertiary rocks of the region was studied. In 

 general, as noted above, the Franciscan forms the basement 

 upon which the marine sediments of the Merced group were 

 laid down and upon which the lavas, volcanic ashes and brec- 

 cias of the Sonoma group were outpoured. The Franciscan 

 is, however, in fault contact with the Petaluma formation of 

 upper Miocene age in the vicinity of Lakeville. This fault, 

 whose trace is indicated upon the geologic map of the Peta- 

 luma Quadrangle, is apparently an ancient one, as the lavas 

 of the Sonoma group rest upon the Franciscan south of Tolay 

 Creek as well as upon the Petaluma formation immediately 

 north of that stream at about the same elevation. Evidently, 

 no pronounced movements have taken place along this line of 

 weakness since the close of the Miocene. For convenience this 

 line of weakness will be referred to as the Tolay Fault. 



Monterey Group, Middle Miocene 



This group is excellently exposed in the Point Reyes Penin- 

 sula where it is the most important formation from an areal 

 viewpoint. Anderson's mapping and description of this ter- 

 rain have been borrowed from his excellent paper in order 

 that the reader may grasp certain problems which are only 



18 Lawson, A. C, U. S. Geological Folio, No. 193. 



