AflCHEAlSr IN FELCH MOUNTAIN DISTRICT. 393 



Quartz occurs in small and often jDartly rounded areas, some of which 

 have a very clastic appearance. Except as stated below, it is generally free 

 from inclusions of the micas, which surround and terminate against it in 

 such a way as to indicate that it crystallized the earlier. It is often crowded 

 with fluid and gas inclusions, and an occasional grain bristles with radiating 

 clusters of rutile needles. Minute crystals of magnetite are also frequently 

 inclosed. The inclusions of all kinds are frequently groiiped in roughly 

 oval areas near the centers of the grains, while between the nuclei and the 

 wandering perimeters the qu:artz is relatively free from inclusions. 



Biotite, varying in color from dark brown to light yellowish green, is 

 the predominant mica. It occurs in irregular plates, generally much larger 

 than the quartz ; the great abundance and uniform alignment of these plates 

 produce the schistose structure. As already stated, it includes and is there- 

 fore younger than the quartz generally, but it is also found, though rarely 

 and always in very minute plates, included in the small quartz grains which 

 are so abundant in the fresh microclines. The latter occurrences belong to 

 an earlier generation than that of the larger biotites. The chief interest 

 attaching to the biotite is in its alteration under the attack of the weather. 

 The iron separates out along the cleavages in little spheroidal drops and 

 flattened plates, which are red and translucent, but not qiiite of the deep 

 color of hematite. Doubtless they contain some water, and are possibly 

 close to gothite in composition. Between the red globules the biotite sub- 

 stance becomes paler, its pleochroism diminishes, and double refraction 

 increases, and finally, in a slide containing no basal sections, it can not be 

 distinguished from muscovite. The separated ferric oxide remains in the 

 mica, and while the rock remains firm does not travel and stain the other 

 constituents. In these stages the slide contains a very faintly colored 

 bleached biotite, which is sprinkled through and through with the little 

 dots of bright red iron ore. 



Muscovite is not very abundant. It is sometimes intergrown with the 

 large biotites, and occurs under similar conditions, but it chiefly comes in 

 little ragged inclusions in the secondary microcline. In the form of aggre- 

 gates of sericite it composes the macroscopically conspicuous pearly micas, 

 and also is an abundant constituent, and sometimes the only representative, 

 of the partly absorbed and older feldspars included in the microcline. 



Microcline is always a secondary mineral, and is present in variable 



