340 GEOLOGY OF THE EUREKA DISTRICT. 



the orthoptnacoid, as in perthite; iu one section nearly in the plane of the clinopina- 

 coid the striping crosses the basal cleavage parallel to the direction of the other 

 piuacoid, and the angles of extinction for the main crystal and the included lamellae 

 are about 7 and 18, respectively, on the same side of the basal cleavage ; angles which 

 correspond to orthoclase and albite in such a section. A zonal structure is com- 

 mon to many individuals, and may be observed in the fresher crystals, even in the 

 hand specimens, without the aid of a lens. There are no characteristic inclusions, 

 but particles of the associated minerals are frequently met with, especially near the 

 margin of the crystal. The plagioclase is very similar iu occurrence to the orthoclase, 

 being characterized by the abundance of striations produced by multiple twinning, 

 mostly in one direction, like that in albite, but also in a second direction nearly at right 

 angles to the first, like that in pericline. Exactly how many species are present has 

 not been determined optically, but it is certain that labradorite is one of them, as the 

 highest symmetrical extinction angles reach about 30. It is in general quite free 

 from inclusions; nevertheless, in some of the plagioclase crystals from widely different 

 parts of the dike there are minute, colorless, rectangular bodies always parallel to 

 the twinned lamell<e, that at once remind one of the glass inclusions characteristic of 

 andesitic plagioclase. Their nature, however, is doubtful, for though without influence 

 on polarized light in most instances, they appear in others to affect it slightly, and 

 besides are without a gas-bubble, from which it seems probable that they are not 

 glass, but possibly feldspar. The substance of the feldspar is sometimes perfectly 

 fresh and transparent, at others clouded by minute, irregularly shaped particles, that 

 reflect incident light and appear white. The orthoclase when further altered is filled 

 with brilliantly polarizing shreds of colorless potash-mica, arranged parallel to three 

 directions in the crystal. Calcite is noticed iu the partially decomposed plagioclase, 

 the decomposition in general setting in from the outside of a crystal and traversing it 

 in the most irregular manner. 



The next essential mineral to be mentioned is biotite. It is universally present, 

 but in varying quantities. In the thin sections from this body it is mostly altered. 

 The fresh mineral is in poorly denned, six-sided crystals of a dark brown color, 

 with strong absorption parallel to the basal cleavage, the basal sections yielding 

 apparently uniaxial interference figures, with a negative character, but sometimes 

 showing a small angle between the optic axes. It is quite free from inclusions, but 

 occasionally carries small crystals of zircon and apatite, and more frequently titanic 

 iron; in one instance (12) it is surrounded by grains of iron oxide, and in another (22) 

 by hornblende. The alteration that has taken place iu most of the sections appears 

 to be a bleaching out of the brown color, leaving a yellow or light green, brilliantly 

 polarizing mica, with faint pleochroism, which is generally filled with slender acicular 

 crystals of a yellowish brown color arranged in Hues intersecting at 60, besides 

 larger and stouter crystals very perfectly developed, which have a high index of 

 refraction and seem to belong to the tetragonal system. From their close association 



