342 KRA^IM— SERPENTINES OF THE CENTRAL [June 6. 



This would explain the occurrence of serpentine veins in sand- 

 stones, a feature observed in the Coast Ranges and construed by Dr. 

 Becker^^ as showing the conversion of the sandstone into serpentine. 



While nearly all of the secondary minerals can be found in 

 every serpentine locality in minor quantities, local conditions favor 

 the accumulation of some few. Of minerals which are of a second- 

 ary nature serpentine itself is the most important. It is represented 

 by its two varieties, antigorite and chrysotile. 



The antigorite shows under crossed nicols as an aggregate of 

 irregularly distributed minute bands and scales which have low 

 interference colors, usually gray with a bluish tint. Due to the irre- 

 gularity of distribution, extinction is compensatory. 



Bastite is a variety of antigorite and is microscopically prominent 

 in coarse prismatic phenocrysts, pseudomorphous after enstatite or 

 bronzite. It has a prominent pinacoidal (lOo) cleavage and is dis- 

 tinguished from the pyroxenes in that it is soft and is readily 

 scratched by a knife. 



Under the microscope these pseudomorphs are irregular in out- 

 line, and consist of coarse bands of serpentine in parallel arrange- 

 ment. A cross fracture is frequently observed, often at right angles 

 to the fibers, but it may have any angle. The whole is somewhat 

 pleochroic from colorless to light green. Extinction is parallel to 

 the lines of prominent cleavage and the slow ray is parallel to the 

 elongation. When alteration has been complete, the characteristic 

 low bluish-tint interference colors of antigorite are exhibited. Using 

 thin uniform sections these colors are raised as the degree of altera- 

 tion becomes less and approach those of enstatite or bronzite. Thin 

 cleavage plates give an interference figure. 



The chrysotile consists of an aggregate of parallel fibers filling 

 the numerous seams which traverse the rock in all directions. It 

 has parallel extinction, the parallel position of the fibers and the 

 bright interference colors, usually red or blue of the second order, 

 make it easily distinguishable. ]\Iicroscopically it is prominent as 

 silky veins which often reach considerable thickness, and are the 

 well-known serpentine asbestos. 



" Ibid., p. 277. 



