Vol. 4] Enopf-Tlvelen. — Geology of Mineral King. 



245 



bring out the schistose structure of the rock. Its orientation 

 around granulated areas of feldspar surrounding unshattered 

 nuclei emphasizes the mode of reduction of the feldspars by 

 attrition. 



An important valuation of the gneissic quartz porphyry is 

 a strongly schistose facies showing abundant patches of drawn- 

 out feldspar and a few augen of vitreous quartz. Upon the sil- 

 very white foliation planes of this schist are curious aggregates 

 of sheared biotite disposed in irregular orientation. They are 

 more or less rectangular in shape and cause the rock to simulate 

 the appearance of the so-called "picture schists." In thin sec- 

 tion a few shattered phenocrysts of quartz can be observed. 

 However, one was noted which was entire and of uniform polar- 

 ization tint, free from inclusions, and with but slight shadowy 

 extinction. Its shape is such as to suggest its derivation from 

 the dihexahedral form, but can not be conclusively demonstrated. 

 Its boundaries are not geometric lines, but are crenulate. A 

 peculiarity in the distribution of the interference colors sug- 

 gests a well healed gash running across the phenocryst. In con- 

 vergent light the section shows an eccentric uniaxial cross of a 

 weakly birefringent mineral. The homogeneity and relatively 

 large size of the quartz crystal, and the sharp plication of the 

 lines of sehistosity caused by it, accentuate strongly its pheno- 

 crystic character. A few kaolinized and shattered phenocrysts 

 of orthoclase also occur. The biotite is mostly flaky and is local- 

 ized in bands. The groundmass is very fine grained and consists 

 of quartz ( f ) more or less interwoven with sericite. Magnetite 

 and zircon occur as accessories. Other slides illustrate in equally 

 remarkable fashion the anomalous behavior of the quartz. The 

 presence of nearly undisturbed quartz phenocrysts in a highly 

 sheared, fine grained ground is an immediately striking feature 

 and calls for an explanation. 



The serieitic quartz porphyries included under 15 and 17 

 differ widely from the porphyries described in the preceding 

 paragraphs. The various facies of serieitic quartz porphyries 

 appear to grade imperceptibly from one to the other, and often 

 to become highly feldspathic and deficient in quartz. The com- 



