I9I0.] COAST RANGES OF CALIFORNIA. 323 



Of the many hand specimens collected from this region one is of 

 especial interest (Plate XXXIV, Fig. 2). It was taken from an 

 outcrop on the western bank of Rocky Creek, and about one half 

 mile south of the Shamrock Aline. It is a bastite pseudomorph 

 after a pyroxenite and has beautifully retained its characteristic 

 structure. The individual crystals are of a lighter green tint. The 

 whole mass appears somewhat dull with now and then a glittering 

 surface. The rock is massive, tough, due to the interlocking 

 crystals and is easily scratched by a knife. 



The microscope reveals that alteration has not been complete. 

 Prismatic crystals of enstatite with prominent cleavage at right 

 angles to the elongation of the fibers are still present. The slide 

 shows bastite, some antigorite and a few minute veins of chrysotile. 



Of interest are numerous transparent grains irregularly dis- 

 tributed with a tendency to concentrate along the bastite bands. 

 They present a medium high relief and are isotropic. Sometimes 

 these grains approach octagonal outline, but mostly they are of oval 

 shape. Optically they behave like spinel, the index of refraction 

 of which, being lower than that of methylene iodide, differentiates 

 it from garnet. Plate XXXV, Fig. 3 gives an idea of the spinel. 



The same mineral was found in a pseudomorph after a web- 

 sterite from Mount Diablo, where it is of a secondary nature, since 

 it was not observed in fresh rock. This similarity of condition 

 makes it safe to assume that it is a secondary mineral here also. 



The total quantity of this variety of serpentine is a small one. 

 The outcrop is surrounded by the kind of serpentine common in 

 this area, which is altered to a considerable extent. 



The nature of the primary rock is indicated by a thin section 

 made of fresh serpentine in which remnants of olivine, enstatite and 

 diallage are found. Other minerals present are picotite, chromite 

 and magnetite. 



The predominating rock with which the serpentine is associated 

 is a medium-grained grayish-yellow sandstone interbedded with 

 shales. As one follows the road from Lower Lake to Knoxville and 

 approaches the serpentine, one sees enormous thicknesses of it ex- 

 posed. The dip is high, usually near 90 degrees. The angles are 

 not constant. They vary considerably, sometimes within a few 



