422 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



spherulites. Sometimes they occur together, but usually independently. 

 The tourmaline is recognized by its strong absorption and by its other 

 optical properties. Its color is brownish green to colorless. The mica 

 is green, and also yellowish brown to reddish. The green mica is easily 

 confounded with the tourmaline, but may be distinguished by the direction 

 of the absorption. The tourmaline and mica are idiomorphic, and must 

 have crystallized just before the outer portion of the small spherulites did, 

 and also before the tridymite and quartz in which they lie. They are con- 

 fined to the region of these interspherulitic spaces and are not found 

 scattered indiscriminately through the spherulites. A mineral which is 

 probably garnet occurs in the same manner as tourmaline and mica, and 

 forms irregular grains, which are colorless and isotropic and have a high 

 index of refraction. Occasionally the more coarsely crystallized quartz and 

 feldspar in the interspherulitic spaces are traversed by opaque material 

 segregated in curved layers, suggesting the structure of Eozoon. In PI. 

 LV, fig. 2, these streaks appear to-be continuations of the lines of trichites 

 that traverse the spherulites and originally marked the flow planes in the 

 lava, They have been displaced by the crystallization of the larger crystals 

 of feldspar and quartz. 



In the banded lithoidal rhyolite the layers, often microscopic in size, 

 consist of minute spherulites alternating with various modifications of the 

 crystallizations just described. These are really holocrystalline, with gas 

 cavities, a phase of miarolitic structure. The size of the cavities is some- 

 times considerable; and may be observed megascopically, giving rise to 

 planes of weakness in the rock along which it splits. The microspherulitic 

 layers may also grade into glassy layers, so that holocrystalline and glassy 

 layers alternate with one another in some modifications of rhyolite, notably 

 in certain parts of Obsidian Cliff. This is illustrated in PI. LVI, fig. 4, 

 representing a section across laminated lithoidite. Microspherulitic layers 

 alternate with layers containing comparatively large feldspar crystals with 

 quartz and tridymite. In one layer the coloring matter is segregated in 

 streaks, like ribs. 



MICROGKANULAE STRUCTURE. 



While the glassy and spherulitic structures are those most commonly 

 found in the rhyolites of the Yellowstone Park, several modifications of 



