SERPENTINE 



49 



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FIGURE 36. Typical boulder strewn surface developed on area under- 

 lain by blocky serpentine in the lower parts of the New Almaden 

 district. This kind of surface results from the erosion of the 

 sheared serpentine matrix leaving the unsheared blocks as residual 

 boulders. 



canic rocks of the Franciscan group. In the lower 

 parts of the district the blocky masses of serpentine 

 support, in addition to sparse grass, a growth of scat- 

 tered bushes and a few struggling oak or bay trees, 

 whereas similar bodies in areas of greater altitude 

 generally support a dense, locally impenetrable, growth 



-< . 





FIGURE 37. View of margin of a serpentine mass in the low foothills 

 of the New Almaden district. The short but pronounced steepen- 

 ing of slope at the contact is typical. Note in lower right the 

 water trough that utilizes the small flow from a contact spring ; 

 such springs and seeps are fairly common along the lower margins 

 of serpentine masses. 



of manzanita bushes. By the unusual gray-green 

 color of the manzanita leaves it is possible in many 

 places to recognize serpentine masses from a distance 

 even where none of the boulders project above the 

 bushes, and in a few places it is possible to delineate 

 the masses fairly accurately by outlining the man- 

 zanita thickets. 



The boulders that weather from the blocky serpen- 

 tine range in diameter from about 6 inches to more 

 than 20 feet, but the majority are between 1 and 4 

 feet in length. The largest boulders are mostly sub- 

 rectangular, the smaller ones nearly spherical. In 

 fresh exposures the boulders are coated with the 

 sheared matrix material and are shiny and smooth, 

 but in most places they are weathered and have rough 

 dark-colored lichen-covered surfaces. The roughness 

 of the surface is due to differential weathering. The 

 bastitic pseudomorphs after pyroxenes are more re- 

 sistant than the serpentine derived from olivine and 

 protrude from the surface. Most surfaces are parti- 

 tioned by a rectangular network of narrow veins of 

 antigorite which are perpendicular to the surface of 

 the boulder and penetrate the rock for only about 1 

 inch; such veins are easily weathered, producing a 

 surface that appears intricately cracked. Larger veins 

 of chrysotile asbestos cut completely through the rock : 

 these veins are more resistant and stand out on weath- 

 ering, as do also some thick veins of porcelaneous ser- 

 pentine. About 1 inch below the surface of the boul- 

 ders, where the antigorite veinlets pinch out, there is 

 commonly a zone of fractures or veins, and on intense 

 weathering these tend to open, causing some spalling 

 of the crust. Some boulders show parallel banding 

 due to the concentration of bastite in layers 1/2 to 3 

 inches thick, and where the rock is weathered, these 

 layers stand in relief. 



Sheared serpentine forms a large part of many of 

 the smaller serpentine bodies, particularly the more 

 linear ones, and some of the smaller bodies consist 

 entirely of the sheared variety. Such masses, except 

 where they are silicified, afford poor outcrops or are 

 not exposed at all except in artificial cuts. Generally, 

 however, they are covered with a distinctive soil, 

 which is black and very sticky when wet, but when 

 dry. is dark-gray, hard, and traversed by wide polyg- 

 onal joint cracks. Fortunately, this soil contains mi- 

 nute shreds of sheared serpentine, for without these 

 shreds it could easily be confused with the soil rest- 

 ing on the black alta, which is derived from the sedi- 

 mentary rocks of the Franciscan group along the 

 margins of the serpentine. Landslides are common in 

 the larger masses of sheared serpentine, and along the 

 upper scarps of these landslides are found the best 



