I9I0.] COAST RANGES OF CALIFORNIA. 335 



Black Mountain and one about four miles south of Saratoga. All of 

 these are mapped in the Santa Cruz folio of the U. S. G. S. 



Small pyroxenite bodies which consist of diallage are frequent in 

 the Black Mountain areas. A feature of the Redwood area is tre- 

 molite secondary after serpentine, and talc after tremolite. 



The similarity of the serpentine itself to that of the Coyote area 

 makes further comment unnecessary. 



VI. The Mount Diablo and Mount Hamilton Serpentines. 



A Iherzolite serpentine and a websterite are found at Mount 

 Diablo in Contra Costa County. According to H. W. Turner^^ the 

 area as a whole is dike-like, its length about five miles and its 

 average width less than one half mile. The peridotite has largely 

 been converted into serpentine, but specimens are still found which 

 contain olivine, enstatite and diallage. 



The pyroxenite is made up of bronzite and diallage, and is 

 most prominent on the southwestern part of the area, occupying ap- 

 proximately one quarter of a square mile, but is also found at var- 

 ious other parts of the area. 



The dike nature of the serpentine is best shown where the 

 Arroyo del Cerro and its branches cut across the area east of the 

 western fork of Pine Creek. The serpentine here varies in width 

 from a few feet to 150 feet, and is enclosed in dark calcareous 

 shales containing at several points near the serpentine " AiiccUa 

 Piochi," with a strike and a dip about that of the shales which 

 enclose it. 



North of the serpentine area is found a diabase and the Knox- 

 ville sandstone, to the south it is bordered by metamorphosed sand- 

 stones. 



A gabbro crops out north of the point where the serpentine 

 dike crosses Bagley Creek. Between this gabbro and the serpen- 

 tine lies like a body of Ancella-he2ir'mg shale, and up to present time 

 no genetic connection between the two has been demonstrated. 



The following analyses are given : 



"H. W. Turner, "The Geology of Mount Diablo," Bull. Geol Soc. Am., 

 II., 383, 1891. 



