APPENDIX B. 



MICROSCOPICAL PETROGRAPHY OF THE ERUPTIVE ROCKS OF THE EUREKA 



DISTRICT, NEVADA. 



BY JOSEPH PAXSON IDDINGS. 



CHAPTEE I. 

 GRANITE AND PORPHYRY. 



The representatives of this division of eruptive rocks from the Eureka District 

 are but few in number, ami bear a very close resemblance to one another, being all 

 quartz- orthoclase rocks. They are composed of the same minerals, having in addition 

 to the quartz and orthoclase a triclinic feldspar with biotite and hornblende in varying 

 quantities. They are granite, granite-porphyry and quartz-porphyry. 



Granite. Of the many varieties of crystalline rocks found within the small area 

 of the Eureka District, granite plays but an insignificant role, and is represented 

 by only four thin sections from the exposure south of Ruby Hill; of these, 1, 2, and 

 3 show a fine grained rock of uniform texture, with the characteristic granitic struct- 

 ure. None of the individuals of quartz and feldspar have crystallographic outlines, 

 but are irregularly shaped by reason of the mutual penetration of adjacent grains. 

 The essential components of the rock are quartz, feldspar, hornblende, and biotite. 

 the accessory minerals being titanite, iron oxide, apatite, zircon, and allanite, besides 

 secondary minerals resulting from the decomposition of the first, which are chlorite, 

 calcite, quartz, epidote, and hydratcd oxide of iron. The rock is, therefore, an 

 amphibole granitite. The most abundant primary constituent, quartz, occurs in irreg- 

 ularly shaped grains which, together with its inclusion of portions of all the other 

 primary minerals, shows it to have been the last to crystallize. It occasionally occurs 

 in porphyritical grains. The only characteristic inclusions are minute fluid cavities 

 with very small moving bubbles. It shows the phenomena of irregular optical orien- 

 tation resulting from mechanical deformation. The feldspar is for the most part altered, 

 but the fresher sections show it to be both orthoclase and plagioclase in nearly equal 

 proportions. They both have a fine zonal structure; the former is frequently in Carls- 

 bad twins, the latter in multiple twins, after the albite and sometimes also after the 

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