MICROSTRUCTUKES IN EHYOLITES. 423 



the rock exhibit a more or less evenly granular microstructure, which may 

 be microcrystalline or microcryptocrystalline. In places it is hypidiomor- 

 phic ; in others the grains are so minute as not to be distinctly discernible. 

 In some cases this structure accompanies a distinct flow structure which is 

 marked by opaque grains. In others no flow structure is noticeable. It 

 appears to be a primary crystallization in many cases, even where a flow 

 structure is observed, when it must have been a crystallization accompany- 

 ing the final solidification of the rock. But it is undoubtedly secondary in 

 some instances, which are not of frequent occurrence among the extremely 

 little altered rhyolites of the Yellowstone Park. 



RELATIONS OF THE VARIOUS MICROSTRUCTI7RES TO ONE ANOTHER 



IN THE ROCK MASS. 



The various microstructures which have been described in detail in a 

 systematic manner occur together in quite different combinations. Pumice 

 is found associated with dense glass, and grading into it. Usually the 

 surface of a flow is pumiceous, as at Obsidian Cliff, but the easily abraded 

 pumice has undoubtedly been carried away by glaciatioh in most places. 

 It is not, however, a necessary accompaniment of obsidian flows, as is shown 

 by such lava streams in the island of Lipari. It was probably more 

 frequent in the Yellowstone region. Pumiceous and compact glass is often 

 intermingled over the surface of the plateaus. The dense glass is usually 

 microlitic and often spherulitic, but in no case has pumice been found to 

 contain spherulites. They occur in vesicular obsidian. 



The various kinds of microlitic glass may be found in close association 

 with one another, and all the different forms of incipient crystallization and 

 spherulitic aggregation may be found in a single specimen of rock, even in 

 one thin section. It oftener happens that a particular form of spherulitic 

 growth prevails throughout a considerable mass of rock. But in such 

 occurrences as the rhyolitic flow at Obsidian Cliff there are parts of the 

 mass in which there is great variability in the microstructure within short 

 distances. In a great many localities it is clearly seen that the thick flows 

 of rhyolitic lava were pumiceous and glassy at the top and glassy for 

 some distance downward, usually with megascopic spherulites. They then 

 became lithoidal by the development of microspherulitic structure, as in the 

 litlioidite at Obsidian Cliff. The microcrystalline to microcryptocrystalline 



