208 GEOLOGY OF TONOPAH MINING DISTRICT. NEVADA. 



RELATIONS OF PTRITE AND SIDERITE. 



The relations of the siderite to the pyrite in these rocks have been carefully 

 studied. In some cases the siderite has been observed distinctly pseudomorphous 

 after the pyrite. Often the two exist side by side in such a way as to suggest 

 contemporaneous deposition, pyrite showing usually, and siderite frequently, some 

 characteristic forms (PI. XXIII). In observing the alteration of these minerals 

 from ferrotnagnesian crystals it has been repeatedly noticed that the carbonate had 

 more intimate relations with the original crystal than did the pyrite, the carbonate 

 occurring all through the decomposed mineral, while the pyrite was distinctly 

 confined to the outer zones. 



ALTERATION OF SODA-LIME FELDSPAR TO QUARTZ AND SERICITE. 



The feldspar phenocrysts, which are sometimes fresh enough to be determined, 

 are typically andesine-oligoclase, though sometimes they become more calcic. 

 Labradorite occasionallj' occurs. The}' are usually partly or completely altered. 



The alteration to adularia is one of the most commonly observed changes, but 

 hardly so common as that to quartz and fine muscovite. These two last-named 

 minerals frequently form a pseudomorphous aggregate in the space occupied by the 

 original feldspar. With increasing alteration the outlines of these pseudomorphs 

 become more and more indistinct and finally indistinguishable. Even within the 

 veins, however, careful observation may often succeed in distinguishing the traces 

 of these original crystals in the highly silicified mass, for sometimes they are marked 

 by quartz that is relatively coarser grained than that in the groundnuts*, and 

 consequently they appear slightly lighter in transmitted light. These two processes 

 of alteration of the feldspar, either to adularia or to quartz and sericite, although 

 present in the same rocks, are not very commonly associated in the same specimens 

 and appear to be distinct. Occasionally the feldspar is altered to kaolin, as described 

 later. 



ALTERATION OF SODA-LIME FELDSVAR TO ADULARIA. 



The alteration of the soda-lime feldspar to adularia can be observed in all 

 its stages in different rock specimens. The alteration proceeds along the edges 

 and the cleavage cracks of the crystal, so that the brightly polarizing andesine, 

 somewhat turbid from decomposition, becomes reticulated with the fresh glassy 

 adularia, which shows markedly lower polarization colors (PI. XXIII). Character- 

 istic complete or incomplete crystals of adularia with rhombic outline frequently 

 form within the older crystal. In some cases the alteration is completely carried 

 out and the feldspar is completely pseudomorphosed to adularia, whose perfect 

 crystal outlines give the idea of a fresh primary crystal, but whose optical 



