PETROGRAPHY— EHTOLITES AND TRACHYTES. 489 



most frequent accessory minerals, and Zirkel has recently proved the pres- 

 ence of augite. 



Trachyte has been divided, according to the character of its feldspars, 

 into sanidin-trachyte and sanidin-oligoclase-trachyte; but, as the tri clinic 

 feldspar or plagioclase has been shown by the microscope to be almost 

 always present with the sanidin, this distinction cannot be considered very 

 exact, nor can the line be sharply drawn been the two varieties. It being 

 the same with the Black Hills trachytes, these names are only used as con- 

 venient to indicate the predominant composition of the various rocks. The 

 silica percentage in these sanidin-trachytes varies from about 56 to 65 per 

 cent. 



Owing to the occurrence of augite mentioned above, a new variety, 

 augite-trachyte, has been introduced by Zirkel. 



The sanidin occurs generally in tabular or columnar crystals, varying 

 greatly in size, but having a bright luster and the characteristic structure, 

 being traversed by numerous fissures. The crystals are very frequently 

 composed of colorless zones, one conrpletely surrounding the other in the 

 form of the crystal, but each varying a little in such a way that the external 

 shape is different from the interior commencement. This formation is easily 

 observed in the thin sections under the microscope, when the zones, often 

 of extreme delicacy, are distinctly seen by reason of their different colors 

 in polarized light. This structure of sanidin is much oftener met with in 

 trachyte than in rhyolite. For examples see Plate II, Fig. 1, and also the 

 section of the rock [141-2]. 



As inclusions, sanidin contains in great abundance particles of color- 

 less or brownish glass, hornblende microlites, grains of magnetite, &c, with 

 which the larger ciystals are often crowded. Occasionallv, these interposi- 

 tions are arranged in zones as the crystal has increased in size, and particles 

 which appear to be only fragments of sanidin are seen from this grouping 

 of the inclusions to be crystals having an irregular contour, the interior 

 zones showing the sharper form. Again, there is sometimes an opaque 

 kernel of these inclusions, around which a more transparent rim of feld- 

 spar has been deposited. Portions of the groundmass are also found as 

 inclusions in the sanidin crystals. Fluid inclusions were formerly consid- 



