COLORED RHYOLITIO GLASSES. 405 



cause, as already intimated. This double condition of refraction does not 

 appear on thin rods taken from a parallelly fibrous pumice. There is only 

 the adjustment of refraction produced by the stretching. 



Colored glasses free from microlites are rather more frequent among 

 the rhyolites of the Yellowstone Park than colorless ones. They are largely 

 pumiceous, or are collapsed pumice. The colors in thin section are dark 

 seal brown to yellow, less frequently orange and red. In most cases the 

 coloring matter can not be resolved into discrete particles, but appears to 

 have been in solution. It seldom happens that the glass is of uniform color 

 throughout the extent of a rock section. It is usually variegated, brown 

 and yellow, and either or both of these occur with colorless glass. Brown 

 glass pumice occurs on the west shore of Yellowstone Lake and elsewhere. 

 In it the brown color grades into yellow along the margin of gas cavities 

 and along the surface of rods; in some places the contrast is sharply 

 marked. It is not due to a thinning of the glass. The color is different in 

 kind, and its distribution is very similar to that of the interference phe- 

 nomena in the colorless pumice. In the brown glass, however, no double 

 refraction is noticeable. A change from brown to colorless takes place in 

 the same manner, and in some cases the transition is from brown to yellow, 

 and then to colorless. It is evident that in these instances the change of 

 color is connected with the inflation of the glass into pumice. In other 

 cases a cause is not so apparent. But the fact is obvious that the change 

 has been from darker to lighter color, from brown to colorless. It is often 

 observed that the inclusions in phenocrysts are brown, while the ground- 

 mass surrounding them is lighter colored or colorless. In one case (1909) the 

 glass inclusions are brown, and the bays of groundmass in the phenocrysts 

 are brown and yellow, the groundmass as a whole being yellow, with some 

 patches of brown scattered through it. These appear to be remnants of a 

 once brown glass almost wholly changed to yellow. A collapsed brown 

 and yellow glass pumice is shown in PI. LI, fig. 1. It carries phenocrysts 

 of quartz and sanidine, with glass inclusions and "bays." In a few cases 

 colorless glass rods are seen to have yellow margins, as though the change 

 had been from colorless to yellow. 



In numerous cases a pumiceous character is entirely wanting. The 

 mass is compact glass, but it consists of irregularly shaped streaks and 

 patches of different color. These twist and curve about one another and 



