404: GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



of the glass fibers and films, that the inflated glass mass settled back upon 

 itself, or collapsed, after the escape of much of the gas. This is possible 

 from the fact that after a magma has been rendered pumiceous it may 

 still remain viscous before its temperature is reduced to the point of solidi- 

 fication. It has been observed during the artificial fusion of obsidian that 

 rhyolitic pumice will retain its expanded form at a temperature at which it 

 is viscous enough to be easily compressed or penetrated by a rigid body. 

 Hence, in a moving stream of rhyolitic lava, portions which have been 

 inflated to pumice may be forced while yet plastic into more compact 

 masses by the movement of the lava, and they may be expected to exhibit 

 some indications of their former pumiceous condition. When we remember 

 the enormous extent of many of the streams of rhyolite in this region, we 

 may easily imagine the formation of pumice over the surface of an intensely 

 heated area of lava, thus permitting of its subsequent welding. 



An example of collapsed pumice is found in that which occurs on the 

 plateau forming the continental divide southwest of Madison Lake (1942). 

 It is a colorless glass free from microlites and with phenocrysts. It is 

 partly pumiceous, but the bubbles are small, elongated, and greatly twisted, 

 as are also the glass fibers and films, which curve about one another in 

 endless complication. Their form is indicated by faint lines, which may 

 mark films of gas, or may be the boundary between glasses with slightly 

 different refraction and color. Parts of the glass are faintly yellowish, and 

 most of it exhibits the double refraction common to perlitic glass. A perlitic 

 structure is present in portions of the glass. The direction of vibration 

 of the slowest-traveling ray is normal to the perlitic cracks and to the 

 surface of the fibers, which should also be the direction of the least stress, 

 the fracture having relieved it. It is observed in some cases that there is 

 a distinct margin to the cross sections of some of the glass rods, and this 

 margin has a faint color and different refraction from that of the center. 

 It is more strongly refracting parallel to the direction of vibration of the 

 slowest ray, normal to the surface or boundary of the rod. The central part 

 of the rods and perlitic masses exhibits an opposite state of strain. Thus, 

 within the glass rod the direction of vibration of the slowest ray coincides 

 with that of the axis of the rod — that is, the direction in which it was 

 stretched. But in the marginal portion the direction of vibration of the 

 slowest ray is normal to the first; it must therefore be due to some other 



