EHYOLITIC GLASSES JSTEAELY FEEE FEOM MICEOLITES. 403 



by the rhyolites of the Yellowstone National Park, we shall consider, first, 

 those most nearly amorphous — that is, almost completely glassy; and sub- 

 sequently, those with more and more crystallized mineral constituents, or 

 those in which the mineral crystals have attained their most advanced 

 development. This method of procedure will separate for the time neigh- 

 boring portions of one rock whose connection can be pointed out subse- 

 quently, but it has the advantage of allowing a comparison of all similar 

 structures and the selection of those which offer the best illustration of the 

 probable explanation of the cause or origin of the structure under discussion. 



GLASSES FREE OE ALMOST FEEE FEOM MICEOLITES. 



Glasses absolutely free from microlitic crystallization are seldom met 

 with, but many of them have so few microlites that they may be classed 

 with the former as glasses free or almost free from microlites. Such glasses 

 may be colorless in thin section, or colored to a greater or less degree. 

 The colorless glasses free from microlites are in almost every instance highly 

 pumiceous, so that the glass solidified in thin rods or films, and it is found 

 that those portions of the mass not so highly inflated carry abundant micro- 

 lites. It must therefore have been the sudden expansion of the inclosed 

 vapors, and the consequent chilling of the magma, that prevented the 

 microlitic minerals from crystallizing before the magma solidified. In such 

 cases it is evident that the magma reached the surface of the earth in a 

 wholly amorphous condition, except for the phenocrysts, which may or 

 may not have been developed. In one instance a pumiceous glass is full of 

 microlites, which must have crystallized before the rock became pumiceous. 



Colorless pumice free from phenocrysts forms the upper portion of the 

 obsidian flow on the plateau southeast of Obsidian Cliff. It is slightly 

 microlitic. Pumices free from microlites, but with more or less phenocrysts, 

 occur on Madison Plateau south of Madison Canyon, on the edge of the 

 plateau west of Yellowstone River near the mouth of Otter Creek, on the 

 west shore of Yellowstone Lake south of Bridge Creek Bay, and in other 

 localities. The gas cavities in these pumices are sometimes microscopic, 

 and are generally elongated, spindle-shaped or drawn out to long thin tubes. 

 Occasionally they appear to contain a small amount of liquid. In many 

 cases the cavities are comparatively large, as in ordinary pumice. In some 

 instances it is evident, from the confusedly twisted and curved arrangement 



