Vol. XI] DICKERSOX—PT. REYES AND SANTA ROSA QUADRANGLES 553 



large, glassy feldspar phenocrysts can be seen, but no quartz. The 

 ground mass appears to be noncrystalline. Microscopically, this rock is 

 seen to consist of numerous rather poorly formed phenocrysts of potash 

 and soda-lime feldspars enclosed in a fine grained ground mass, com- 

 posed mostly of glass. No ferromagnesian mineral is present, the only 

 iron-bearing mineral being occasional cubes of magnetite and flakes of 

 hematite, the feldspars frequently being stained with the latter. The 

 most abundant phenocryst is sanidine. Its frequent straight extinction 

 and absence of repeated twining served to distinguish it from the 

 plagioclase present. It is very abundant, and sometimes occurs in well 

 terminated crystals, but usually in broken fragments, frequently badly 

 kaolinized. A relatively small amount of plagioclase occurs, of which the 

 highest extinction angle observed on (010) was 10.5°. This would in- 

 dicate either albite or oligoclase. The ground mass is very fine grained, 

 and under the high power is seen to be composed of minute fragments 

 of feldspar, apparently sanidine, intimately mixed with unindividualized 

 glass. 



Determined solely by its optical properties, this rock would be called 

 a trachyte, since no quartz phenocrysts were observed. A silica deter- 

 mination, however, showed it to contain 72.13% Si0 2 . Hence is classed 

 as a Rhyolite. A similar rock from above the Sonoma Tuff on the 

 west side of Wooden Valley, Napa County, yielded 72.36% SiO-. 



Detailed studies on the west side of Sonoma Mountain 

 show that the history represented by the different horizons 

 of the Sonoma group is highly complex. Along the Moun- 

 tain School road in the Santa Rosa Quadrangle, four differ- 

 ent beds of tuff separated by basaltic flows were recognized. 

 That erosion intervals occurred between the basalt flows and 

 ash deposits was evidenced by irregular contacts and streaks 

 of basaltic gravels and sandstones between the major tuff 

 strata and the basaltic flows. The Neohipparion gidleyi beds 

 at Lawlor's Ranch indicate that some of these intervals were 

 of considerable duration. These beds are fairly well exposed 

 on Lawlor's Ranch just below the ranch house, between the 

 600 and 700 foot contours, one mile S. 30° E. from the Moun- 

 tain School, about 6 miles N. E. of Petaluma at an old coal 

 prospect. About 60 feet of sandstone, lignite, and car- 

 bonaceous shale, and freshwater chert is found in a small 

 canyon. Considerable sliding has taken place and it is diffi- 

 cult to tell where the freshwater chert fits in the section. 

 Beneath a lava flow, a stratum of sandstone occurs, then ap- 

 parently a three- or four-foot seam of coal resting upon car- 

 bonaceous shale, and possibly the freshwater chert next ; and 

 finally a coarse sandstone is found resting upon compact tuff. 

 The thickness of these members was not observable. The coal 

 as reported by Mrs. Thompson, a daughter of Mr. Lawlor, 

 Avas of good quality when mined and she also stated when 



