GLOBULITIC EHYOLITIO GLASSES. 407 



yellow, grading in places into globulitic brown surrounded by a colorless 

 zone with a yellow margin, or the central portion is colorless with swarms 

 of minute grains, the margin being as in other cases. The minute g-rains 

 are bright yellow in incident light, and almost opaque by transmitted light. 

 With high magnifying lens they appear to be partly transparent, but no 

 double refraction is noticeable. 



In other cases these globulites are represented by microlites, many of 

 which are opaque g*rains, apparently magnetite, together with others which 

 will be described. The transition of yellow and brown colored glass streaks 

 into globulites and microlites is well shown in the colored obsidian from 

 Obsidian Cliff, represented by fig. 1 of PI. XVI in the Seventh Annual 

 Report of the United States Geological Survey, in which the curved and 

 contorted bands of color and streams of trichites produce grotesque shapes 

 in thin section, like imitations of various forms of organic nature. From 

 this it appears that the tones of yellow, orange, red, brown, and black are 

 due to different states of oxidation of the iron in the magma, which may be 

 crystallized out as magnetite and sometimes subsequently oxidized to the 

 ferric state, as shown by the red color of many trichites in incident light, 

 or the iron oxide may be disseminated in minute particles whose precise 

 character is not determinable, or the coloring matter may be in solution. 

 In some parts of Obsidian Cliff the red and black streaked glass is brecci- 

 ated, the angular fragments being welded into a compact mass, as shown 

 in PI. LI, fig. 3. An extremely delicate banding- is shown in PI. L, fig. 4, 

 caused by alternating streaks of colorless and red and yellow globulitic 

 and trichitic glass. The folding of the streaks is well shown. They pass 

 through faint spherulitic growths. 



A frequent microstructure is one produced by thin films of globulites 

 and particles like dust, which appear to be scattered in planes through 

 colorless glass. The films curve and fold about one another in the most 

 intricate manner, producing the effect of thin veils. This structure appears 

 to have resulted from the welding together of fragments of g'lass, as in the 

 case of certain colored glasses already described. This veil structure is 

 often retained after the whole mass has become crystalline, and serves to 

 indicate one of the phases through which the magma has passed. 



