406 GEOLOGY OP THE YELLOWSTONE NATIONAL PAKK. 



appear like a perfectly welded mass of strips or ribbons and irregular frag- 

 ments of variously colored glass. In some cases their shape closely 

 resembles that of fragments of jmmice pressed together and welded (PI. L, 

 figs. 1,2, and 3). In others it appears as though such fragments had been 

 drawn out and twisted by a movement of the mass (PI. LI, fig. 2). 

 Undoubtedly this has been the case, but it is doubtful whether all the 

 streaked and variegated glasses have passed through the process of inflation, 

 collapse, and welding with subsequent flow. However, the distinctly out- 

 lined and strongly contrasted streaks and ribbons of variously colored 

 glass, are with difficult}' explained in any other manner. Another possible 

 explanation will be given in connection with the development of microlites. 

 This streaked arrangement of colored glasses has been called " eutaxitic" 

 structure. 



GLOBULITIC GLASS. 



The colored glasses are in many cases crowded with minute dots or 

 particles, which, are the pigment, the glass itself being colorless. Such 

 glasses are said to be globulitic. The connection between the globulitic 

 and colored glasses is well shown in certain banded and streaked colorless 

 and orange glasses, as well as in brown, yellow, and colorless ones. In the 

 former it is observed that the orange-colored bands in places become 

 globulitic with minute orange or yellow particles. As these particles 

 become more distinct and larger they are farther apart and are usually 

 darker colored, in extreme cases becoming opaque black grains, dissemi- 

 nated through colorless glass. Brown glass in like manner passes into 

 colorless glass with brown globulites, which may be opaque in the extreme 

 forms. Such globulitic glass may appear bluish by transmitted light when 

 not in focus, owing to dispersion of the light. It is evident that the glob- 

 ulitic pigment is a segregation, and eventually a crystallization, of the 

 coloring matter, which sometimes permeates the glass as though in solution. 

 There is a distinct contraction or condensation of the coloring material, for 

 bands which grade from colored glass to globulitic, and into those with 

 crystalline grains, become thinner and narrower, often resulting in a film of 

 particles, or in a contracted belt noticeably smaller than the undifferentiated 

 belt. In some cases of welded pumice of variously colored glass the 

 general color is yellow or orange (2058). Some fragments are colorless at 

 the center, with yellow margins, while the larger fragments are either 



