QUAETZ-POEPHYRY. 345 



tioMS iiud shows a slight iiiifrh> between the optic axes, but it is mostly altered to a 

 light yellow chlorite, through which are scattered grains of a yeUow, liighly refracting 

 mineral, resulting fioiii the alteration of iluienite and corresponding to leucoxeiie, 

 besides very small, sharply deliiied, colorless crystals, apparently epidote. The 

 crystals of hornblende are very well developed, showing the prismatic and both 

 l)inacoidal faces, together with the base and pyramid. The individuals are compara- 

 tively large and broad, with the characteristic cleavage. The color is light brown, 

 frequently green along the margin. The pleochroism is strong from brown to yellow, 

 c=: b>a. The highest extinction angle measures 19°. Along the cleavage crack red 

 oxide of iron is sometimes deposited, and, though for the most part fresh, a few are 

 completely altered to an irregular aggregate of fibrous chlorite and hydrous oxide of 

 ir(m, through which ruu cohjrless needles with an extinction angle of 17°, which are 

 ])robably actinolite. The accessory minerals are magnetite, with some ilinenite partly 

 altered to leucoxene, a very little titanite, and a large amount of apatite, both in 

 short crystals and also in extremely long, slender, colorless, hexagonal prisms, occa- 

 sionally broken and bent, but generally perfectly straight, although one measures 

 0-44°"" long by 0'007.5""" wide, or is sixty times as long as it is broad, which indicates 

 that the mass commenced to crystallize after all motion in it had ceased. 



Quartz-Porphyry.— Unfortunately the only body of rjuartz-porphyry found in the dis- 

 trict is completely decomiiosed. It occurs in the vicinity of the Bullwiiacker miue 

 and is represented by thin sections 3L, 32, and 3.'5, which have essentially the same 

 structure, though the first is full of pyrite and the second and thii-d are discolored by 

 hydrous oxide of iron. It is closely related to grauiteporphjTy, having apparently a 

 microgranitic groundmass; but a thin film of isotropic glass is detected between the 

 grains along the thinnest edge of section 31, and colorless glass is tbund included in 

 the macroscopic quartz grains, whose quartz-porphyry liabit is further evinced by intru- 

 sions of groundmass, small amount of fluid inclusions, some of which have salt cubes, 

 and by the absence of liquid carbon dioxide. The quartz shows a well developed 

 rhombohedral cleavage, especially in section 32, and is the only primary mineral except 

 apatite and zircon remaining unaltered. A small amount of feldspar is indicated by 

 patches of a colorless, aggregately polarizing substance, probably kaolin. The mica 

 occurs in comparatively large crystals, much elongated in the direction of the vertical 

 axis, which have been altered to a mass of confused lamina' of colorless i)otash-mica, 

 calcite and red oxide of iron. The groundmass also is crowded with shreds of potash- 

 mica, but it seems ])robable that in both of its occurrences it is of sei-ondary origin. 

 Sections that have the outline of hornblende crystals are filled with (talcite and ferrite, 

 and quite large deposits of calcite with very distinct rhombohedral cleavage have filled 

 cavities in the rock. Iron is ])resent as magnetite and the hydi'ous oxides and as 

 ilmenite and pyrite, the latter in comparatively large crystals, including portions of 

 the groundmass. Apatite and zircon occur in very small quantities. 



