SUMMARY. 457 



pyroxene, and I cull this rock pseudodio) He. No nietamorpliic rocks Iiave 

 been found in place which carry olivine. Glaucophane occurs in l)otli tlie 

 pseudodiabase and the pseudodiorite. The quantity of z(»isite in tliese rocks 

 is very variable and in some cases is so great that with feldspar it forms almost 

 the entire mass. The schistose metamorphics, not including'' phthanites, are 

 all characterized l)y the presence of glaucophane. In every case but one, 

 zoisite is associated with the glaucophane in this group and either muscovite 

 or biotite is usually present. 



The phthanites or silicified shales form a very distinct group readily 

 distinguishable from the granular metamorphics. They are usually green 

 or brown and are intersected by innumeralde quartz veins. They cou- 

 tain microscopic" organic remains, and embedded in the (piartz veins or pro- 

 jecting from their walls are often numerous zoisite cr^-stals. All of these 

 rocks are best represented by detailed descriptions of special .examples, for 

 which there is no space here. 



Serpentine in a comparatively pure state occurs throughout the quick- 

 silver belt in irregular areas. As nearly as can be estimated these areas 

 amount to somewhat over one thousand square miles between Clear Lake 

 and New Idria. Serpentine is also one of the mineral constituents of man\' 

 of the altered sandstones and of the granular metamorphic rocks. It is a 

 biaxial variety, often just perceptibly dichroitic, and rarely shows dift'er- 

 ences of tint as great as those characteristic of chlorite. It might be called 

 antigorite if it seemed needful to separate the biaxial serpentines. The net 

 structure so usual, though not invariable, in serpentine formed from olivine 

 has nowliere been detected. Where any considerable quantity of serpen- 

 tine is present it usually shows the now well known grate structure. 



No considerable portion of the serpentine of the Coast Ranges has 

 resulted from the decomposition of olivine. Only in one district have 

 pebbles of olivine gabbro have been found, and these contain a mere trace 

 of serpentine, while the origin of the serpentine has been traced in a great 

 number of cases to rocks containing no olivine. 



Field observations show most conclusively that the great mass of the 

 serpentine of this area is derived from the sandstones, either immediately 

 or through an intermediate granular metamorphic rock. 



