424 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



modifications exist in some cases near the bottom of the lava sheet, but no 

 definite mode of occurrence has been made out, In places the bottom part 

 of the flow is glassy and dense; in places it rests on a bed of pumiceous 

 tuff and is lithoidal; that overlying the rhyolitic tuff on Mount Everts is 

 microcrystalline (1762). The indurated tuff, indistinguishable from the 

 overlying massive lava, exhibits a fragmentary glass structure, with some 

 axiolites, and occasional spherulitic structure independent of the outline of 

 the fragments, while parts of the mass are isotropic. 



LAMINATION AND BANDING. 



Lamination and banding are highly developed in the lithoidal portion 

 of the rhyolitic flow at Obsidian Cliff. They are very generally present 

 in a higher or lower degree in all the rhyolites of the Park. In fact they 

 form one of the commonest characteristics of acid lavas, and are equally 

 uncommon among the basic ones. They are clearly due to the spreading 

 out through flowage of a more or less heterogeneous viscous fluid. Homo- 

 geneous portions of the mass, of whatever shape, will spread out and flatten 

 during the flow of the whole body of lava, becoming thin lenticular layers 

 if the spreading or flow is sufficiently extended. A mass consisting of por- 

 tions which differed from one another in color or composition would 

 become, after spreading out upon the surface of the earth, a body made up 

 of layers of different color or composition, which would wedge out in thin 

 edo-es between one another. A cross section of such a body would exhibit 

 a more or less streaked, banded, or laminated structure according to the 

 original size of the different portions and to the extent of the spreading. 



In the case of the lithoidite at Obsidian Cliff, it can be shown that 

 the cause of the differences in the layers was unquestionably the different 

 amounts of water vapor present in them. For in the lithoidite the 

 layers differ in their degree of crystallization, some being glassy, others 

 microspherulitic, others more coarsely so and porous, while others are 

 microgranular with larger cavities. Some layers have locally developed 

 crystallization in the form of large spherulites and lithophysse. In the 

 obsidian the differences consist in layers of spherulites, large and small, in 

 bands of lithophysse, and in layers abounding in micrographic feldspars, 

 microscopic spherulites, microlites, and trichites — that is, in the different 

 phases of crystallization. Near the surface of the lava flow the laminated 



