APPENDIX R. 



MICROSCOPICAL PETROGRAPHY OF THE ERUPTIVE ROCKS OF THE EUREKA 

 DISTRICT, NEVADA. 



By Joseph Paxson Iddings. 



CHAPTER I. 



GRANITE AND PORPHYRY. 



The represeutativ^es of this division of eruptive rocks from the Eureka District 

 are but few iu number, and bear a very close resembhuice to one another, being all 

 quartzorthoclase rocks. They are composed of the same minerals, havinj;' iu 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 tine grained rock of uniform texture, with the cliar.octeristic 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 allauite, besides 

 secondarj' minerals resulting from the decomposition of the first, which are chlorite, 

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

 amjihibole granitite. Tiie most abundant jirimary constituent, (piartz, occurs in irreg- 

 ularly shai)ed grains which, together with its inclusion of [lortions of all the other 

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

 in porphyritical grains. The only charactertistic inclusions are minute fiuid cavities 

 with very small moving bubbles. It shows the phenomena of ii-regular 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 flue zonal structure; the former is freqiiently in Carls- 

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

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