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University of California. 



[Vol. 2. 



described, the structure, color, transverse partings, and extinction 

 angle all indicating this fact. In some instances, along the cleavage 

 cracks, darker-colored cloudy areas, evidently marking the begin- 

 ning of alteration, may be observed. The dark-colored areas are 

 resolved into a dark brown somewhat opaque ground mass, and 

 veins composed of grains of magnetite. Apparently they represent 

 those portions in which, on account of the relatively large amount 

 of surface exposed, alteration has been most complete. The blue- 

 black surfaces of the rock are due entirely to veins of magnetite. 

 Specimens collected from the same locality show still further devel- 

 opment of the process of serpentinization. 



The chemical investigation of the rock, the results of which are 

 recorded in the third analysis of Table VI, confirm the above theory. 

 The decrease in silica and lime and increase of combined water and 

 magnesia, clearly indicate the progress of the process of serpentin- 

 ization. The specific gravity of the rock is 3.153. 



SERPENTINIZED ANTHOPHYLLITE. 



This occurs in groups of massive boulders, often rounded but 

 more commonly angular, which outcrop at several points in local- 

 ity B, often projecting to the height of twenty feet above the pre- 

 vailing level and suggesting the character of intrusives. The rock 

 is exceedingly tough and compact, and does not fissure readily. 

 The outer surfaces are of an ashy gray color but freshly-exposed 

 portions present yellowish-green cleavage planes or curved surfaces 

 of a light brownish-yellow. Thin flakes of the mineral possess the 

 yellow translucent features characteristic of serpentine, but on the 

 whole the rock possesses few of the more obvious features charac- 

 teristic of that mineral. Microscopic examination shows for the 

 most part a fibrous mineral which is distinguished by its high inter- 

 ference colors, parallel extinction and correspondence of the direc- 

 tion of least elasticity with the cleavage trace. These features all 

 indicate the mineral anthophyllite. In addition there are found 

 considerable amounts of serpentine, grains of magnetite and fine 

 veins of calcite. 



For the chemical examination a mass bounded by freshly- 

 exposed surfaces only was employed. The finely-ground powder 



