344 KRAMM— SERPENTINES OF THE CENTRAL [June 6, 



Trcmolitc was found at two places in connection with the ser- 

 pentines in the neighborhood of the Culverbear group of quicksilver 

 mines in Sonoma County and at the Redwood area of serpentine 

 in San Mateo County. Small lenticular bodies are imbedded in the 

 serpentine. More often they are found as float. They reach a 

 diameter of four to five inches, are usually coated with iron stain, 

 and are exceedingly tough. The mass has a somewhat schistose 

 structure and when broken shows needle-like white crystals with 

 a silky luster. 



The microscope shows an interwoven mass of slender crystals. 

 Some few are of stocky habit but with no definite shape. A cleav- 

 age parallel to the elongation and a cross fracture is present but not 

 pronounced. The extinction is parallel in some sections, in others 

 it varies. The maximum angle measures about 22 degrees. The 

 slower ray is parallel to the elongation of the crystal and sections 

 with parallel extinction show the emergence of an optical axis. The 

 trace of the optical plane lies parallel to the cleavage. Sections 

 cutting at right angles the plane of schistosity show a characteristic 

 amphibole cross section with a pronounced prismatic cleavage of 

 about 124 degrees. 



Inclusions of a dark green mass of antigorite are frequent. The 

 alteration of it into tremolite is plainly visible. It begins first on 

 edges. Bunches of needle-like crystals are tangent to the more or 

 less oval-shaped body. Their higher interference colors contrast 

 sharply with the low birefringence of the antigorite. 



The serpentine is dotted with specks of a brighter color. Under 

 a high-power objective they resolve themselves into radiating bun- 

 dls of fibers of tremolite. The process of alteration is therefore not 

 confined to the boundaries of the mass but is also an internal one. 



Tremolite was also observed in a section from a pseudomorph 

 after a websterite from Mount Diablo. 



Talc is very rarely found in connection with the serpentines. At 

 the Redwood area it was found in place secondary after tremolite. 



Hydromagncsite. — This mineral is a product of decomposition 

 of the serpentine. Local conditions seem to influence its forma- 

 tion, as it is more abundant in some localities than in others. In 

 the Sulphur Creek areas it is abundant. 



