QUARTZ PHEKOOEYSTS. 395 



QUARTZ. 



The phenocrysts of quartz vary in amount and in size in different parts 

 or bodies of the rhyolite. They may be wholly absent or quite abundant. 

 In some cases they are 4 or 5 mm. in diameter; in others, any size less than 

 this, generally from 2 to 4 mm. They are noticeably smaller than the 

 sandines in most instances. Their crystal form is that of hexagonal 

 bypyramids, formed by the equal development of the plus and minus unit 

 rhombohedrons, together with small prism faces, but the crystals are often 

 rounded to a greater or less extent (PI. LI V, fig. 4). They are generally 

 much cracked, the cracks being largely spheroidal, so that the quartzes on 

 rock surfaces usually appear as rounded grains. In some instances their 

 crystal form is preserved and they fall from the rock in perfect double pyra- 

 mids. Owing to the highly fractured condition of the quartz, much of it 

 falls out when thin sections of the rock are prepared, and a false impres- 

 sion may thus be gotten of its relative abundance unless the thin section is 

 compared with the rock. In some varieties of the rhyolite it is in excess of 

 the other constituents, and from this proportion its relative amount decreases 

 until it is entirely absent in some varieties, which may be rich in feldspars. 

 Its microscopical characteristics are much the same in all varieties of 

 rhyolite from the Yellowstone Park. Its substance is colorless and very 

 pure, except for well-defined inclusions of glass, and very rarely crystals 

 of other minerals. Individuals of quartz differ in one rock section both as 

 to outline and as to inclusions. Distinctly idiomorphic crystals occur by 

 the side of rounded ones. Some quartz sections are free from inclusions, 

 while others in the same rock section contain a few or many inclusions of 

 glass and bays of groundmass (PI. LI, fig. 1). The glass inclusions usually 

 occupy pyramidal cavities, or they may be rounded; they are seldom 

 irregularly shaped. They generally contain one gas bubble, whose size 

 appears to vary in proportion to the volume of the glass in any one section. 

 In some instances there are no gas bubbles present. 



The inclosed glass is in some cases colorless, in others brown, as in the 

 illustration just referred to, usually one character or the other prevailing 

 throughout the quartzes of a rock section. But often colorless and brown 

 inclusions occur by the side of one another in the same quartz crystal. It 

 frequently happens that the glass inclosed in the quartz is different in char- 

 acter from that forming the groundmass of the rock, when this is glassy. 



