QUABTZ-POEPHYEY. 345 



tions and shows a slight angle between the optic axes, but it is mostly altered to a 

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

 mineral, resulting from the alteration of ilmenite and corresponding to leucoxene, 

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

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

 pinacoidal 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 

 iron, through which run colorless needles with an extinction angle of 17, which are 

 probably actinolite. The accessory minerals are magnetite, with some ilmenite 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 mm long by 0-0075 mm 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. 



Quaru-porphyry.-Unfortuuately the only body of quartz-porphyry found in the dis- 

 trict is completely decomposed. It occurs in the vicinity of the Bullwhacker mine 

 and is represented by thin sections 31, 32, and 33, which have essentially the same 

 structure, though the first is full of pyrite and the second and third are discolored by 

 hydrous oxide of iron. It is closely related to granite-porphyry, 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 found included in 

 the macroscopic quartz grains, whose quartz-porphyry habit 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 1 of colorless potash-mica, 

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

 mica, but it seems probable that in both of its occurrences it is of secondary origin. 

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

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

 cavities in the rock. Iron is present as magnetite and the hydrous 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. 



