Rapport de l'État minéralogiste . e. Photo de Walter W. Bradley. omhmhmh flBHBHB SffiSSM3|£& ? *** ? Je Wk, i * 1 . - M ^^x HP™ . &Lt ; ? * * ? Jij ? ? :W % : : 4| 1 . Photo n°16. Dans l'ouest de la mine de magnésite Development Company, showingmagnesite plus de 30 pieds de largeur. Photo de Walter W. Bradley. Le Comté de Santa Clara. 191 dérivé est décrit par Hess comme la lherzolite et de péridotite et est composedessentially des minéraux olivine, diopside, et une orthorhombicpyroxene. La serpentine est un minéral produit d'altération des roches naturelles fromsuch. Les roches du pays d'origine sont principalement composés o

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Report of the State Mineralogist . e. Photo by Walter W. Bradley. omhmhmh| SffiSSM3flBHBHB£&? **** ? i wk, i * 1 . - m ^™ HP^x . < ? * * ? Jij ?? :W%:: 4| 1 . Photo No. 16. Stope in mine of Western Magnesite Development Company, showingmagnesite over 30 feet in width. Photo by Walter W. Bradley. SANTA CLARA COUNTY. 191 derived is described by Hess as lherzolite and peridotite and is composedessentially of the minerals olivine, diopside, and an orthorhombicpyroxene. The mineral serpentine is a natural alteration product fromsuch rocks. The original country rocks are composed principally ofsilicates of magnesia (silica, 40 per cent; magnesia, 25 to 40 per cent;alumina and iron variable, usually 5 per cent or more; and a smallpercentage of alkalies). It is natural to assume that magnesite mightbe derived by the decomposition or alteration of these silicates beingdeposited as veins along fissured zones or by replacing the country rock.As a whole, however, the serpentine of this area does not show the. Photo No. 17. Tramway terminal and mine dump of the Western Magnesite DevelopmentCompany. excessive amount of shearing to be seen at many of the other magnesitelocalities. Throughout the dark red soil that covers the area of serpentinecountry rock, especially in the zones that contain the magnesite veins, silica is present in various forms. It occurs, in part, as a white androsy opal, scattered fragments of which are strewn about on the surface, or it may be observed as chalcedonic veins or coatings in the joints ofthe country rocks. The silica associated with the purer mass of mag-nesite in the larger pit is in the form of a pale-greenish granular quartz. The magnesite veins are very irregularly distributed and appear totrend in all directions. The larger developed masses are of mostunusual size, and even the great bodies that have been removed bymining have been taken out in large open caves or chambers, so as to 192 MINES AND MINERAL RESOURCES. give little eviden

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