MODE OF OCCUREEXCE OF SEHPEMTINE. 121 



Near the Golden Gate, at Mt. Diablo, and in many other localities it is veiy 

 abundant. In fact, though the quantitative relations of" the various meta- 

 inorphic rocks vary greatlvin diflferent neighborhoods along the quicksilver 

 belt, all varieties are to be found in almost every district. Serpentine is 

 commonly, but not always, intimately associated with psf udodiabase. Tliese 

 rocks stand in such relations to the unrJtered sandstones in very numerous 

 cases that no geologist carefully exjimining the localities could fiiil to con- 

 clude that they are modifications of the sandstone. This was substantially 

 the conclusion at which Professor Whitney arrived. Knoxville aflbrds ad- 

 mirable opportunities for studying this connection. Highly inclined strata 

 strike into serpentine areas in such a manner as wholly to preclude the sup- 

 position that the serpentine represents an earlier mass ; one side of an an- 

 ticlinal fold is serpentinized, while the other is unaltered and carries excellent 

 fossils, and there are clear cases of transition through altered sandstones. 

 Mt. Diablo affords equally favorable opportunities for determining the age 

 and relations of the serpentine. In many other localities the relations of 

 the serpentine to unaltered rocks are evident, although it is seldom that tlie 

 age of these unaltered rocks can be immediately determined. Tlie reason 

 for assigning them all to the Neocomian will be given in Cliajiter V. 



Field observation makes it clear that in most cases the transformation 

 to serpentine began along cracks in sandstone or in rocks resulting from 

 the alteration of sandstone and worked toward the centers of the frag- 

 ments tluxs separated from one another. Where this process is incomplete, 

 partially rounded nuclei of rock retaining the sandstone habitus are to be 

 seen, divided by a net-work of serpentine. Sucli exposures remind one of the 

 appearance presented vmder the microscope by olivines in process of con- 

 version into serpentine. Sometimes, but not often, the serpentine assumes 

 a radial form, the fibers being normal to tlie surface of the nucleus. Pro- 

 fessor Whitney observed such an instance at New Idria ; I found some 

 very beautiful ones at Knoxville. Two cuts illustrating such occurrences 

 will be given in the description of the Knoxville district. 



It is not possible from a mere field examination to determine whether 

 the serpentine results directly from tlie action of solutions upon sandstone or 

 whether the sedimentary rock first becomes crystalline and is subsequently 



