Vol. 4] 



Knopf— Thelen 



, — Geology of Mineral King. 



247 



serieite was noted closely hugging a feldspar half way round its 

 perimeter. In another instance a sheaf-like aggregate of serieite 

 was found separating two feldspars in near juxtaposition. The 

 feldspars are largely orthoclase, though the plagioclase character 

 of some few is barely recognizable on account of advanced kao- 

 linization. An insignificant amount of biotite occurs, as does 

 also a little chlorite. The "round is a fine mosaic of quartz and 

 is often sericitic. A single large fragment of quartz, showing 

 strain shadows, differentiated itself from the ground, and ap- 

 peared to represent the remnants of a phenocryst, but the evi- 

 dence was not decisive. Apatite in needles, and magnetite in 

 quite large aggregates, were relatively abundant. A little epidote 

 and a piece of tourmaline were also found. 



The rock called a tuff under formation 11 is of grayish- 

 green color and is dotted with rather numerous, small turbid 

 white phenocrysts in a very fine grained groundmass. The folia- 

 tion is rude, though very distinct. Under the microscope the 

 porphyritie feldspars are found largely untwinned, though a 

 few show very narrow striations and small extinction angles 

 against the lamellae. The original character of the feldspars is 

 indicated by the circumfluent schistosity ; by their occasional 

 strong idiomorphism and by the frequent peripheral granulation 

 zones ; and by their homogeneity and freedom from inclusions of 

 the minerals of the groundmass. In certain instances the sharp 

 abutment of the feldspars against the cryptocrystalline ground, 

 and the absence of granulation products seem to show in the 

 case of rounded corners evidences of magmatic corrosion. 



The matrix consists of biotite flakes and some chlorite, dis- 

 seminated throughout a limpid, feebly polarizing paste. Mag- 

 netite occurs as accessory, and occasionally apatite needles are 

 included in the feldspars and more frequently lie scattered in 

 the groundmass, where they show marked evidences of cataclastic 

 phenomena. 



It is apparent that the microscopic diagnosis is not conclusive 

 as to the massive or pyroclastic origin of this rock. The field 

 relations, however, show that a continuous gradation exists be- 

 tween this type of rock and others whose clastic origin is more 



