ALTERATION OF THE EABLIER ANDESITE. 213 



Bischof" notes that albite occurs in quartz veins in gneiss in Sweden, and 

 F. A. Genth described it in pyritiferous gold quartz veins in California, 6 and it 

 has been noted as a common occurrence by subsequent observers/ 



It seems to the writer to be unquestionable that waters that deposit albite 

 without orthoclase in a vein are different from those which deposit orthoclase 

 without albite, and that the difference must consist in part in the relatively 

 greater quantity of soda in the waters in the first case -and of potash in the 

 second. The many observed instances in the earlier andesite at Tonopah of 

 complete pseudomorphs of adularia, quartz, sericite, etc., after soda-lime feldspars 

 show a process of replacement (not leaching and concentration), the soda and lime 

 being removed and potash and silica introduced. The waters which accomplished 

 these changes thus must have had abundant potash as well as silica in solution. 



STUDY OF TYPICAL SPECIMENS. 

 MICROSCOPIC DESCRIPTIONS. 



For the purpose of estimating more accurately the changes which have been 

 described as observed microscopically, a number of analyses were made and studied. 

 The specimens selected, arranged in their natui'al order, were as follows: 



1. Earlier andesite (4O8) from Imoer part of Siebert shaft. Dense dark -green 

 rock, Siebert shaft, Mizpah mine, 670 feet from surface. Contains scattered 

 phenocrysts of rather small size in a fine microlitic groundmass, showing flow 

 structure. The microlites in the groundmass are chiefly feldspar. A little zircon 

 and apatite are present. Quartz grains also occur, of which some may be original. 



Among the phenocrysts the feldspars are prominent. A determination in 

 another similar specimen near the same locality showed the species to be andesine- 

 oligoclase. They are largely altered to calcite with a little quartz. Abundant 

 pseudomorphs after hornblende, in which no trace of the original mineral remains, 

 consist of dark blue-green chlorite (ripidolite) with some specular iron. The 

 hornblende cleavage is still visible in the pseudomorphs. Pseudomorphs after 

 biotite consist of fine muscovite, with a little calcite and hematite. 



2. Earlier andesite (358) from.Tonopah and California shaft. Green, but much 

 lighter than No. 1. Shows relatively sparse and small phenocrysts in a fine 

 microlitic groundmass, with much felty devitrified glass. Apatite is abundant. 

 Secondary chlorite occurs throughout the groundmass. 



The feldspar phenocrysts have the optical characters of andesine, and are 

 only slightly attacked by decomposition. The ferromagnesian minerals are 



Bischof, Gustav, Chemische Geologic, vol. 2, p. 412. 



i> Genth, F A., Am. Jour. Sci., 2d series, vol. 28, p. 249. 



c Ransome, F. L., Description of Mother Lode district: Geologic Atlas U. S., folio 63, V. S. Geol. Survey, 1900, p. 8. 



