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



sufficiently coarse-grained to enable the lath-shaped feldspars of the 

 ground mass to be readily seen with the naked eye. Scattering pheno- 

 crysts 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 universally twined on the albite law, and 

 rounded grains of augite, the structure being the common one called by 

 Rosenbusch "Intersertal". The feldspar phenocrysts, measured by the 

 common method of symmetrical extinctions on the albite twining 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 twining lamellae, was evidently cut 

 parallel to the albite twining-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 AbsAn 5 . 



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 feld- 

 spar, is very abundant. It seems to have come from some exterior 

 source as an infiltration. Flow structure 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 pres- 

 ent. 



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

 anticline near the contact at Mark West Springs is very similar in ap- 

 pearance to the rock above described from the west contact. It is a 

 dark greenish-black, heavy rock, rather too coarsely crystalline for a 

 basalt, with scattering phenocrysts of feldspar. 



Microscopically, also, it is similar. It is somewhat fresher, and con- 

 tains much less hematite. By Michel Levy's statistical method, the feld- 

 spars gave a maximum angle of 43.5°. This indicates a basic labradorite 

 of a composition somewhat more basic than Ab 3 An 4 , or nearly AbiAm. 

 Augite occurs sparingly as phenocrysts up to .38 mm. in length. These 

 crystals are rounded and corroded as though acted upon by the magma 

 prior to consolidation. The abundant augite in the ground mass occurs 

 in rounded grains lying between the laths of feldspar in the "Intersertal" 

 structure of Rosenbusch. The feldspar laths are short and stout, and 

 invariably twined on the albite law. The augite is of the usual lavender- 

 gray color. It appears to be altering to chlorite of a dark green shade, 

 which stains the rock freely. No olivine was observed in this slide. A 

 slight flow structure was observed. No glass was recognized. A careful 

 determination of the silica contents of this rock gave 65.13%. 



This tuff (Sonoma) is a fragmental rock made up wholly of the vol- 

 canic material, and characterized by containing numerous fragments of 

 pumice, in size from very small grains up to an inch or more in length. 

 Two silica determinations made on the pumice, from two localities in 

 Santa Rosa Valley, gave respectively 61% and 63% SiO=. Hence it is 

 andesitic in character. 



The rock is usually very light in color and in weight, and, where well 

 exposed, forms a conspicuous feature of the landscape. Certain fine 

 grained varieties of it are easily worked into blocks, which make very 

 good building stone where great strength is not required. 



A specimen (St. Helena Rhyolite) from the top of Mount Saint 

 Helena showed the following characteristics : 



Microscopically, it is a very light colored, almost white, rock, oc- 

 casionally slightly reddish from iron stains, notably lacking in ferromag- 

 nesian minerals. It has a rough, trachytic-like surface. Numerous 



