Vol. 4] Osmont. — Geological Section of Coast Ranges. 



63 



Microscopically it is a heavy dark greenish black rock, very 

 similar in appearance to the last one described. It is evidently 

 crystalline in character but with few phenocrysts showing to 

 the naked eye. 



Microscopically it is seen to be made up of numerous, but 

 small, crystals of feldspar and pyroxene embedded in a ground 

 mass of feldspar microlites and augite grains in the "Tnter- 

 sertal ' ' structure of Rosenbusch. The maximum extinction angle 

 of the feldspar phenocrysts was 41°, hence it is a medium labra- 

 dorite. The largest crystals of feldspar observed measured 

 6 mm. in length, the largest of pyroxene 5 mm. These pyroxene 

 phenocrysts proved to be orthorhombic. They were distinguished 

 from the augite of the ground mass by their straight extinction, 

 lower double refraction, and pleochroism. The latter, however, 

 is very weak, and suggests enstatite. The color of the fresher 

 crystals in ordinary light is a greenish lavender. The larger 

 ones, however, are badly altered along the cleavage planes to 

 hematite and a green mineral of weak double refraction, prob- 

 ably serpentine. 



Augite is plentiful in the ground mass in small, short prisms 

 and rounded grains. The rest of the mass seems to be made up of 

 microlites of bytownite feldspar. No glass was certainly identified. 

 Magnetite in cubes, and in grains scattered through the ground 

 mass, is abundant. Also much serpentine and chlorite is present 

 from the alteration of the pyroxenes. 



The feldspars of the above described rocks vary from a 

 medium to an acid labradorite, and this fact, taken in connec- 

 tion with the abundance of augite and occasional presence of 

 olivine and enstatite, would lead one to call them basalts. But 

 their silica contents is invariably high, averaging close to 65%. 

 Hence they must be classed as pyroxene-andesites. The glass in 

 the ground mass must evidently be very acid. Becker* probably 

 referred to these rocks when he spoke of the "older andesites. " 

 He classed them as pyroxene-andesites. Similar rocks are de- 

 scribed by Palachef from the Berkeley Hills. 



*Quieksilver Deposits of the Pacific Coast. Mon. XIII, U. S. Ct. S. 

 tThe Berkeley Hills. Lawson and Palache. Bull. Dept. Geol., Univ. of 

 Cal., Vol. 2, No. 12, p. 41 2. 



