60 



University of California Publications. 



[Geolocy 



Pablo sandstone and to the supposed Orindan beds below, points 

 to its being of late Pliocene age. 



Petrography. — A specimen of this rock from beneath the 

 Sonoma Tuff near the contact on the west limb of the anticline 

 near Mark West Springs showed itself to be macroscopically a 

 dark, heavy rock, varying from dark greenish black to brown in 

 color, according to degree of weathering, and sufficiently coarse 

 grained to enable the lath-shaped feldspars of the ground mass 

 to be readily seen with the naked eye. Scattering phenocrysts of 

 feldspar and of olivine occur up to 4 mm. in length. 



Microscopically this rock is coarse in texture, consisting of a 

 few large phenocrysts of labradorite and olivine scattered 

 through a rather coarsely crystalline ground mass, made up 

 chiefly of labradorite feldspar in well shaped laths almost uni- 

 vei'sally twinned on the albite law, and rounded grains of augite, 

 the structure being the common one called by Rosenbusch "Inter- 

 sertal." The feldspar phenocrysts, measured by the common 

 method of symmetrical extinctions on the albite twinning plane 

 (101), gave a maximum extinction angle of 37.5°. According 

 to Michel Levy, this angle corresponds to a labradorite of about 

 the composition Ab 3 An 4 . One crystal, rhombic in section, with 

 good cleavages parallel to (001) and (100), and showing no 

 twinning lamellae, was evidently cut parallel to the albite twin- 

 ning plane (010). It gave an extinction angle measured against 

 the trace of (001), of 22°. The extinction fell in the acute angle 

 of the rhomb, making the sign negative. This corresponds to 

 labradorite of a composition between Ab 3 An 4 and Ab 3 An-. 



Small crystals and grains of magnetite occur in some cases 

 formed around the ends of the feldspar laths, never included in 

 them. Hematite in flakes and irregular patches, and as a mere 

 stain discoloring the feldspar, is very abundant. It seems to have 

 come from some exterior source as an infiltration. Flow struc- 

 ture is very noticeable, the feldspar laths of the ground mass 

 being drawn out in more or less parallel lines, and wrapped 

 around the ends of the phenocrysts. A little glass is present. 



A specimen from beneath the Sonoma Tuff on the east limb of 

 the anticline near the contact at Mark West Springs is very simi- 

 lar in appearance to the rock above described from the west 



