396 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The inclosed glass may be colorless and that of the groundmass strongly 

 colored, yellow, brown, or red; or vice versa, It often happens that the 

 inclosed glass remains such, though the groundmass of the rock becomes 

 holocrystalline. In a few instances the colorless glass inclusions contain 

 minute idiomorphic crystals, which are probably augite, for in one case a 

 comparatively large crystal exhibits the optical characters and green color 

 of augite. One glass inclusion contains twenty of these crystallites. In 

 another section two colorless glass inclusions contain curved trichites. No 

 fluid inclusions have been observed. 



In the great majority of cases each quartz phenocryst is a single 

 individual, with perfectly uniform optical orientation, and no indication of 

 internal strain or displacement, except around the glass inclusions in a few 

 instances. In several rock sections some of the phenociysts of quartz con- 

 sist of two or three quartzes grown together, with more or less divergent 

 orientations. They are parti} 7 - idiomorphic and partly rounded. 



In most of the rhyolite sections the quartzes occur entirely isolated 

 from phenociysts of other minerals With the exception of sanidine, it 

 is almost never observed in juxtaposition with other minerals, a common 

 occurrence among the phenociysts in andesites, where several kinds of min- 

 erals are often crystallized in clusters. The phenociysts of quartz in this 

 rhyolite almost never inclose fragments or crystals of magnetite, zircon, 

 augite, plagioclase, or sanidine, so that the relative period of crystallization 

 of these minerals with respect to quartz would be a matter of conjecture 

 were it not for the fact that intergrowths of quartz and sanidine in direct 

 connection with the phenociysts of these minerals occur in more than one 

 modification of this rhyolite. The intergrowth possesses the well-known 

 micrographic structure, which is often strongly and beautifully developed. 



This micrographic intergrowth is of two kinds, or rather it is the 

 result of two different phases of crystallization. In some cases it belongs 

 to the period of crystallization in which the phenociysts of quartz and 

 sanidine were produced; in others it was formed when the groundmass 

 of the rock crystallized. The latter corresponds to its more common 

 occurrence in certain quartz-porphyries and granite-porphyries, and will be 

 referred to again in connection with the description of the groundmass of 

 the rhyolite. The micrographic intergrowth which belongs to the first 

 period of crystallization in the rhyolite is specially well developed in the 



