96 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The feldspar in all eases is lime-soda feldspar, with the usual poly- 

 synthetic twinning. In sections the forms of the feldspar crystals are lath- 

 shaped, rectangular, and tabular, the general habit being tabular parallel to 

 the clinopinacoid. Zonal structure is pronounced. Their range appears 

 to be from labradorite to oligoclase, the former prevailing in the more basic 

 dike rocks, the latter in the more siliceous varieties rich in biotite. Primary 

 inclusions of glass or of other minerals are scarce. Secondary inclusions 

 are more numerous, and are gas cavities or needles of amphibole. The 

 feldspars are more distinctly idiomorphic than the hornblendes, and are 

 sometimes inclosed in hornblendes. More rarely hornblende is inclosed in 

 feldspar. 



Primary hornblende phenocrysts are generally idiomorphic, but not 

 always. In the prism zone the unit prism (110) and clinopinacoid (010) 

 are well developed. Terminal planes are seldom observed. Twinning is 

 common, parallel to the orthopinacoid (100). The color varies from brown 

 to green, through reddish brown, greenish brown, and light brown, brownish 

 green, and olive gray, sometimes with a reddish tint approaching violet gray. 

 The olive gray and violet gray are common in many hornblendes of these 

 dike rocks. The pleochroism is, then, olive gray || c, olive brown || ]j, light 

 brown || a. The absorption is C>tr>a. The color is sometimes irregu- 

 larly distributed in the crystal, the darker shades being usually in the 

 center, but zonal structure is rare. In the less siliceous rocks of this series 

 the hornolendes are somewhat darker colored, approaching chestnut brown. 

 There are no characteristic inclusions. When associated with biotite the 

 two minerals are so intergrown as to suggest synchronous crystallization. 

 In some cases biotite is inclosed as a secondary mineral. The hornblende 

 is in various stages of preservation, sometimes fresh, sometimes partially or 

 completely altered. The usual alteration products are chlorite and epidote, 

 with calcite and quartz. Sometimes compact hornblende has been changed 

 to fibrous, "reedy" amphibole. 



Biotite forms six-sided plates, occasionally twinned parallel to the 

 cleavage plane. It is dark reddish brown with normal absorption. It is 

 sometimes partially bleached, the light-colored spots containing bundles of 

 rutile needles. It may be completely altered to chlorite and epidote, with 

 calcite and quartz. Pyroxene was not found in an unaltered condition, and 

 was only identified by its form. The iron oxide is probably magnetite, with 



