WHITE PORPHYRY. 325 



Microscopical The essential constituents of the White Porphyry are plagioclase, 

 orthoclase, and quartz, developed in a remarkably uniform-grained mass, in which lie 

 occasional crystals of one or more of the same minerals. Orthoclase seems to pre- 

 dominate, but never very greatly, and the chemical analysis confirms this view. Com- 

 pared with the Mount Zion Porphyry, it is found that plagioclase occurs also in microlitic 

 forms, but less alnmdantly, and in some of the more compact modifications may be wanting. 

 Biotite, which was present in the Mount Ziou rock, has never been seen in any of the nu. 

 merotis specimens of White Porphyry collected, nor was it ever noticed in the field, not- 

 withstanding the fact that much of the rock seems quite fresh, judging from the condi- 

 tion of the feldspars. As the White Porphyry seems in all other respects to be very 

 closely allied to the variety mentioned, it is to be particularly noted that many of the 

 muscovite leaves are found to contain yellowish needles (rutile?) or stout crystals (ana- 

 tase ?) directly comparable to those resulting from the decomposition, of biotite in the 

 Mount Zion and other porphyries. It is therefore probable, in spite of the singular 

 absence of intermediate alteration products, that a part of the muscovite in the White 

 Porphyry came from biotite. Magnetite is found very rarely, and apatite scarcely 

 more frequently, while zircon in minute, brilliant crystals is quite abundant. 



The size of the grains is sometimes but little below the power of vision of the 

 naked eye, and they might frequently be distinguished were it not for the decom- 

 position of the feldspars. In numerous instances, however, usually in dikes or con- 

 tact specimens, but sometimes in large masses, the texture becomes so fine that it 

 is beyond the power of the microscope to identify separate granules as quartz or feld- 

 spar, and the mass thus becomes cryptocrystalline. In all such cases the structure 

 remains evenly granular, there being no tendency towards a development of indistinct 

 fibrous matter, nor does any portion appear amorphous, or, more correctly, isotropic. 

 A few minute, irregular inclusions are usually visible in the larger quartz grains, some 

 of them being undoubtedly fluid, while the others are not recognizable. No glass is 

 determinate, and the minute, dark interpositions in the feldspar are probably second- 

 ary forerunners of the coming decomposition. 



Alteration. Here, as in the Mount Zion rock, the conditions have favored the 

 production of muscovite from all changeable constituents. Only in comparatively 

 rare cases do calcitc and kaolin appear. Many of the specimens collected are very 

 much altered and show when examined in polarized light under the microscope a 

 number of irregular quartz grains, imbedded in a brilliant, variegated mass of minute 

 muscovite leaves. Little aggregates representing the original rnicrolites of plagioclase 

 penetrate the quartz grains in every direction. It is owing to this decomposition that 

 the quartz is ordinarily invisible in hand specimens, as the muscovite leaves envelope 

 each grain so closely that fracture does not separate them. The leaves of muscovite 

 are so very small that the characteristic luster is seldom detected without close exam- 

 ination. 



Chemical composition of Mount Zion and White Porphyries. Analysis I, below, was 

 made by L. G. Eakins, upon a fresh, specimen of Mount Zion Porphyry from the 

 Little Harry shaft, Prospect Mountain [24a]. Analysis II by W. F. Hillebrand, 

 upon a typical specimen of White Porphyry from the quarry in California gulch at 



