ALTERATION OF THE EARLIER ANDES1TE. ' 215 



may be distinguished, is altered to an aggregate of quartz and sericite, with a 

 little iron oxide. The pseudomorph.s after phenocrysts are frequent and well 

 denned. Numerous ones after feldspar form an aggregate of fine felty muscovite, 

 with a little quartz. Those after biotite consist of muscovite, with a little siderite. 

 Pseudomorphs after hornblende or pyroxene, or both, are barely distinguishable 

 from the groundmass. They consist of fine muscovite (sericite) and quartz, with 

 some siderite, which marks the outlines of the original phenocrysts. In this rock 

 the secondary quartz varies in grain, some areas becoming more coarsely crystalline. 



6. Typical earlier andesite (398) from Mizpah Hill. Hard white rock with 

 small glistening feldspar crystals. This rock has a microlitic groundmass, show- 

 ing flow structure. It has the appearance of being unusually fresh, and fresh 

 striated feldspar can be seen in it. Nests of fine granular adularia and quartz 

 (both secondary) occur here and there. There is a little finely disseminated 

 siderite and limonite. 



The feldspars are mostly altered to adularia. The original mineral has the 

 optical characters of an oligoclase, near andesine. The alteration of this to adu- 

 laria can be seen in all its stages. Polarized light brings out this change strongly, 

 the bright white of the soda-lime feldspar contrasting with the dark gray of the 

 potash feldspar. The latter penetrates the former irregularly and minutely, yet 

 with a fairly high power the characteristic crystal outlines (usually rhombic) of 

 the adularia can be distinguished. The process can be observed in all its stages 

 in different crystals, up to the complete pseudomorph. A little sericite accom- 

 panies the alteration in some cases. Traces of original ferromagnesian pheno- 

 crysts can be determined, but with difficulty. In one case a pseudomorph after 

 probable hornblende was of sericite, with apparently a little adularia and traces 

 of siderite. 



7. Earlier andesite (143) from hanging wall of Mizpah vein, 300-foot level, 

 Mizpah mine. Light gray, nearly white rock, with uneven fracture and dull 

 luster. 



This rock is so much altered as to be hardly recognizable. It consists of an 

 aggregate of quartz and fine muscovite, with small scattered pseudomorphs of 

 hematite after pyrite (the result of oxidation), and some siderite (?). The quartz 

 is irregular and is segregated throughout into areas and little veinlets, which are 

 of coarser grain than the quartz of the less altered rocks, while the muscovite is 

 apparently finer than usual. Original phenocrysts of feldspar are indicated by 

 pseudomorphou > areas characterized by different groupings of the quartz and 

 muscovite and freedom from iron, while others of ferromagnesian minerals are 

 marked by similar differences of grouping and by a relatively greater abundance 

 of the iron minerals. In all cases, however, the decomposition products are 

 similar. In many areas also the vestiges of the phenocrysts have been effaced. 



