66 



University of California Publications. 



[Geology 



ley's Ranch, three miles northeast of Windsor, this same flow 

 seems to occur, resting on the Sonoma Tuff. At Mt. St. Helena, 

 Sacro Canon near Napa, and at other places flows of lava inter- 

 calated with the tuff and agglomerate were ohserved. 



Petrography. — A specimen from Mark "West Springs showed 

 the following characteristics : 



Maeroscopically this is a dense dark greenish black rock, in 

 which a few large phenocrysts of feldspar and of olivine can be 

 seen with the naked eye. The ground mass does not appear to 

 be crystalline. 



Microscopically it is a very porphyritic rock, consisting of 

 large phenocrysts of feldspar up to 2 mm. in length embedded in 

 ;i micro-crystalline ground mass of small, well shaped feldspar 

 microlites separated by augite prisms in the common intersertal 

 structure. The microlites appear to be universally twinned, and 

 a maximum angle was obtained on (010) of 33°. A large pheno- 

 cryst also gave 33°. This maximum angle corresponds to a labra- 

 dorite of a composition midway between Ab., An 4 and Abj An r 

 Olivine in prisms and rounded grains is rather abundant. The 

 largest phenocryst observed was about 5 mm. in diameter. They 

 are usually rounded on the corners and altered along the cracks 

 to serpentine. The most characteristic feature of the structure 

 is the sharp distinction between the two periods of consolidation. 

 Large, well shaped crystals of labradorite enclosed in a micro- 

 crystalline ground mass of well formed feldspar laths in inter- 

 sertal structure with augite grains. Flow structure is beauti- 

 fully developed, as shown by the disposition of the microlites 

 around the phenocrysts. A little glass is present in the ground 

 mass. A silica determination gave 43.06% SiO... The rock is 

 a good example of an Olivine-Basalt. 



Another specimen from Greeley's Rranch, about three miles 

 northeast of Windsor, will be described. Here the hills have 

 a thin capping of basalt resting on Sonoma Tuff and gravels. 

 Structurally this basalt appears to the writer to correspond to 

 the flow above described at Mark West Springs. 



Maeroscopically it is grayish black rock, quite scoriaceous, 

 with many vesicles which have been subsequently tilled with opal 

 and iron oxides. The phenocrysts are numerous and easily dis- 



