106 GEOLOGY OF THE COMSTOCK LODE. 



METAMORPHIC DIORITE. 



Slide 295. Amazon dump. 



Typical basaltic variety. — This Tock is of a veiy dark iron-gray color, and is 

 full of bright scaly particles of bisilicates. It is intensely hard and tough. 

 Under the microscope it is seen to be composed chiefly of hornblende and 

 feldspar, but the former is present in great excess, and the feldspar is so full 

 of hornblendic microlites as scarcely to be recognizable. Mica, chlorite, 

 and epidote are also present in considerable quantities. 



The hornblende is of two varieties, green and colorless. The colorless 

 hornblende is wholly undecomposed, shows capitally marked prismatic and 

 clinopinacoidal cleavages. It absorbs light very faintly, but polarizes in 

 brilliant green and purple colors, like augite. Sections parallel to the ver- 

 tical axis show angles of extinction reaching 27°. The green hornblende 

 shows an equally high angle of extinction. It dichroizes strongly between 

 a bright, very slightly brownish, yellow and a dark grass-green. It is often 

 fibrous, and is frequently accompanied by decomposition products. The 

 two species of hornblende stand in the closest relations to one another. In 

 all cases the colorless variety is surrounded by the green ; in cross-sections 

 the white modification appears in polygonal spots in the green ; in the longi- 

 tudinal sections in irregular stripes. Where they occur together in this 

 way the optical orientation of the two is in all cases identical. In fact, the 

 relations are just such as would result from an alteration of the white into 

 green hornblende, and taking into consideration the fact that the green 

 variety alone appears to suffer decomposition into any other mineral, I can- 

 not avoid the conclusion that the case is really one of alteration. The 

 association of colorless and green hornblende is illustrated in Figs. 11 and 

 12, Plate II. All the microlites of hornblende, which are present in great 

 quantities, are green. These microlites are so numerous in the feldspars 

 that the striations are only just perceptible, and the species cannot be satis- 

 factorily determined; indeed, hornblende microlites form the greater part 

 of the rock. 



A considerable quantity of fibrous chlorite occurs between the micro- 



