360 



University of California Publications. 



[Geology 



here and there, probably as a result of the alteration of the 

 chalcocite, but also as individual grains 4-6 mm. across in the 

 granular natrolite matrix. These may represent original chal- 

 cocite grains that have been replaced by solutions percolating 

 through the natrolite and thus becoming charged with silica, but 

 from the freshness of the surrounding natrolite and neighboring 

 chalcocite it may be that some of the copper in the original 

 solutions was held as silicate and separated out primarily as 

 chrysocolla. 



Both of these copper minerals occur side by side with the 

 titano-silicates. 



OTHER MINERALS. 



Amphiboles. 



Some of the drusy surfaces are coated with green or bluish 

 green amphibole needles, and the natrolite and albite crystals 

 often carry them in sufficient quantity to give a greenish or 

 bluish tint. These amphiboles vary from place to place and even 

 the same needle may show different optical properties along its 

 length. 



A common type is actinolite. This has been observed in thin 

 needles growing out into open spaces as a newly formed mineral 

 in the wall rock and in veins. Occasionally it develops as 

 asbestos films or exceedingly fine hair-like bunches. It shows 

 pleochroism in pale green and yellow, the c sometimes having 

 a bluish tint. It gives a distinct sodium reaction in the blowpipe 

 flame. 



Another common amphibole is bluish green in color and 

 shows under the microscope a greenish yellow, b gray violet, 

 c bluish green. The axial plane is transverse to the plane of 

 symmetry and the dispersion is so great that it shows no extinc- 

 tion in plane (010) with white light. The extinction for red light 

 is about 25°-30°, for violet some 5° greater. Its properties would 

 indicate a soda amphibole with considerable iron in the molecule 

 — perhaps intermediate between crossite and crocidolite, but 

 nearer the crossite type. 



A member of the glaucophane group is sometimes present, 

 c blue, b violet, a yellow; c A c about 8°. 



