I9I0.] COAST RANGES OF CALIFORNIA. 333 



What is probably an extension of this area, separated by alki- 

 vium is found on the opposite side of Santa Clara Valley. It follows 

 a group of well-rounded crests which in a wide circular sweep reach 

 to within four miles of San Jose. This area is known as the " Oak- 

 hill," and its crystalline rocks are described by Carey and Miller.^* 



The serpentine here is of a dike-like nature intrusive into radio- 

 larian charts and sandstones of Franciscan age. It is variable in 

 character. A massive phase shows glistening phenocrysts of dial- 

 lage in a dark-green ground mass of serpentine which grades into 

 a diallagite. The structural and mineralogical composition of the 

 serpentine indicates that it is derived from a peridotite containing 

 olivine, diallage and magnetite, with enstatite but sparingly present. 



Associated with the massive green serpentine, which is predomi- 

 nant, is found the slickensided facies and the " mottled " serpentine. 

 The latter consists of the green serpentine to which white colored 

 spots varying in size to a diameter of two inches give a mottled 

 appearance. The white substance may possibly be magnesium sili- 

 cate. A partial analysis of massive conchoidal serpentine is here 

 appended. Plate XXXV, Fig. 5 represents a phase of the mottled 

 serpentine. 



Of much interest is the associated olivine-gabbro which on one 

 hand passes into peridotite and pyroxenite, and on the other into hy- 

 persthene gabbro and norite. An analysis of what may be consid- 

 ered a pyroxenite-peridotite consisting principally of diallage with 

 small quantities of olivine partly transformed into serpentine, and 

 some magnetite was made by the writer and is given below. 



The serpentine of the main area which follows the Los Lagranas 

 ridge does not differ from the one above described. It is intrusive 

 into Franciscan cherts and sandstone. The area is almost destitute 

 of soil and the barren slopes are strewn with boulders of the dark 

 green rock. The outcrops are low and seldom protrude more than 

 three feet above the surrounding soil. The massive facies is pre- 

 dominant. A feature is the abundance of magnesite which is found 

 as float, and also in veins of considerable size. 



Alineralogically the serpentine is a Iherzolite serpentine. The 



mass as a whole appears to be uniform, but variations occur. A 



"E. P. Carey and W. J. Miller, "The Crystalline Rocks of the Oak- 

 Hill Area near San Jose, Cal.," Jour, of GeoL, XV., 160, 1907. 



