380 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



contorted bands of flow. Its base consists of a talus of large blocks. The 

 rhyolite is dark gray, speckled with white and brown. It bears many 

 phenocrysts of white plagioclase and fewer of quartz and sanidine, besides 

 many small rusted crystal's of augite (1964, 1966, 1967). Through the 

 rock are scattered cavities of various sizes and shapes with gray or white 

 walls, which are coated with brilliant crystals of quartz, tridymite, or sani- 

 dine, with opaque crystals of fayalite. The quartzes have been studied by 

 Professor Penfield, who has found them to have a simple but very unusual 

 development. In addition to the common quartz forms — prism and unit 

 rhombohedrons — there are steep rhombohedrons (3032), 3/2 and (0332), 

 3/2, and narrow trapezohedral faces ± 3/2-3/2. 1 



The same rare forms occur on the quartzes in the lithophysas at 

 Obsidian Cliff and in other localities within the Yellowstone Park. The 

 white walls of these cavities vary in thickness from mere lines to an inch, 

 and some of them have a distinctly spherulitic structure with widely gaping 

 centers. They are in part hollow spherulites of a peculiar character, very 

 closely related to the irregular cavities in the rock, which could not be 

 termed spherulites. The outer margin of the hollow spherulites is not 

 sharply defined against the groundmass of the rock, as in most instances. 

 The radial fibration is not recognizable by the unaided eye, and a concen- 

 tric zonal structure is observed in only a portion of them. The most 

 characteristic feature of these hollow spherulites is the occurrence of com- 

 paratively large quartz crystals in two habits, usually in different parts of , 

 the cavity. One form of the quartz consists of stout crystals, seldom over 

 2 mm. in diameter, in one instance 5 mm., very transparent, with a pale 

 smoky to amethystine color. The others are slender white prisms, 10 mm. 

 long. The transparent crystals are often located on a nearly flat side of 

 the cavity, while the white prisms, intersecting in all directions, form a kind 

 of network which occupies the thicker part of the center of the spherulite. 

 The light-colored streaks and more crystalline parts of the rock are punc- 

 tured with minute round holes. Along the cliff to the westward the rock 

 passes into laminated lithoidal rhyolite with open layers incrusted with the 

 same minerals as those in the cavities just described. 



The rhyolite on the second ridge east of Glade Creek is black and 



■Iddings and Pentield, The minerals in hollow spherulites of rhyolite from Glade Creek, Wyo- 

 ming: Am. Jour. Sci., 3d series, Vol. XLII, 1891, p. 39. 



