398 GEOLOGY OF THE YELLOWSTONE NATIONAL PAKE. 



mass, quartz belonging to the final crystallization of the rock having 

 attached itself to the quartz phenocryst with the same orientation (PI. LIV, 

 fig. 4). The original form of the phenocryst can generally be detected bv 

 a faint line of impurities. This structure will be described in connection 

 with that of the groundmass. 



SANIDINE. 



The phenocrysts of feldspar are partly idiomorphic, partly rounded, 

 and often fragmental. They consist of orthoclase and plagioclase, which 

 are present in different varieties of the rhyolite in various proportions. In 

 some cases orthoclase is the only feldspar present; less frequently plagio- 

 clase is the predominant, if not the only, feldspar in porphyritical crystals. 

 The orthoclase usually possesses the habit of sanidine, and may be described 

 under that name. 



Sanidine usually occurs in simple crystals, 5 to 7 mm. long, and also 

 in Carlsbad and Baveno twins, the latter being uncommon. When in 

 unbroken crystals, its usual form is a rectangular prism with truncated 

 edges, furnishing square cross sections and rectangular longitudinal ones, 

 often with the customary terminal planes. It is less frequently in tabular 

 Carlsbad twins. Occasionally it has irregular outlines, caused by rounded 

 intrusions of the groundmass, indicating a partial resorption of the magma. 

 In many cases it occurs in angular fragments, and sometimes the ciystals 

 are split in two and the parts are separated by a stream of groundmass. 

 In some crystals the cleavage planes are well developed and close together; 

 in most instances they are poorly developed and the crystals are traversed 

 by irregular cracks. (PI. L, fig. 1; PI. LI, fig. 4.) The substance of the 

 sanidine is generally very pure and free from decomposition, but inclu- 

 sions of glass and groundmass are common. They are numerous in some 

 forms of the rock, and are partly confined to crystallographic cavities 

 and partly occur in irregularly shaped ones. The glass inclusions are 

 often colorless, often brown, the two kinds occurring in the same crystal. 

 Frequently they contain more than one gas bubble, sometimes as many as 

 thirteen, which is in striking contrast to the single bubble in those in the 

 quartz phenocrysts. Inclusions with several bubbles are generally found 

 in the feldspars in pumice. Crystallites are less common in the glass inclu- 

 sions than they are in those in the quartzes. Sanidine occasional^ incloses 

 crystals of augite and magnetite. It frequently surrounds crystals of plagio- 



