324 



University of California Publications. [Geology 



the tendency to kaolinization and silicification to which the com- 

 mon facies of the porphyry seems so prone. In hand specimens 

 it is a compact, light yellowish gray rock, showing numerous 

 small porphyritic feldspars of a fresh glassy aspect. 



Under the microscope this is seen to consist of a fine holo- 

 crystalline ground mass made up of allotriomorphic and inter- 

 locking feldspars, some of which show lamellar twinning. These 

 are approximately equidimensional and generally free from de- 

 composition products. In this ground mass are imbedded nu- 

 merous phenocrysts of plagioclase, colorless augite, and apatite. 

 The feldspar in sections normal to (010), showing both Carls- 

 bad and albite twinning, have extinctions corresponding to the 

 composition Ab 2 An 3 . 



The rock thus appears to be a pilotaxitic augite andesite. 

 The fresh condition in which this rock occurs promised satis- 

 factory results on analysis. A selected sample was, therefore, 

 placed in the hands of Mr. Herbert Ross of the Ruth Mine, who 

 kindly made a chemical analysis with the following results : 



Si0 2 



63. 



30 



AL.0 3 



17. 



90 



FeO 



1. 



83 



MgO 



2. 



53 



CaO 



10. 



45 



Na : 



2. 



30 



K 2 







PA 





69 



H ; ) . 



co 2 r g 





18 



1. 



100.18 



QUARTZ BLOUT. 



Term Defined. — Perhaps the most remarkable feature of the 

 porphyry is the occurrence, in the areas occupied by the latter, 

 or on their periphery, of large bodies of quartz of somewhat 

 varying character. These bodies of quartz are by no means un- 

 common features of decomposed and mineralized eruptive rocks 

 in Nevada and elsewhere; and the prospectors and miners, who 

 have to deal with them practically, usually distinguish them 

 from ordinary vein quartz by characterizing them as "blow 



