408 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



MIOROLITIC GLASS. 



Most of the rhyolitic glasses in the Yellowstone Park are microlitic. 

 The microlites may be uniformly scattered through the glass, or they may 

 be abundant in alternating layers or bands, or they may occur irregularly 

 in patches and streaks with colored and globulitic glass, as already mentioned 

 They are usually distributed uniformly in massive obsidian, but often 

 exhibit a somewhat parallel arrangement, or, more properly, a slight varia- 

 bility in abundance in parallel planes, which mark the planes of flow or 

 movement within the magma. This may be seen in PI. LV, fig. 1. This 

 variability is more pronounced in portions of the glass which are partly 

 vesicular or pumiceous — that is, where dense microlitic obsidian passes into 

 pumiceous nonmicrolitic glass, as in the upper portion of the lava near 

 Obsidian Cliff and elsewhere. 



The character of the microlites may be surmised from their shape and 

 color in some cases, and from their optical behavior in others, or by tracing 

 similar forms from the minutest to those laro-e enough to be recognized. 

 In this way it is found that they are undoubtedly magnetite, augite, feld- 

 spar, and quartz, and rarely hornblende and hematite, and probably pseu- 

 dobrookite. Magnetite occurs in minute crystals and grains, isolated or 

 attached to prisms and needles of augite. The opaque hair-like trichites, 

 straight and curved, in some cases are separated into rows of opaque grains 

 resembling magnetite. In other cases they form threads on which are 

 strung transparent rhombic plate-like microlites, as in the illustration, PI. 

 LII, fig. 6. Such microlites have a pale-greenish tinge. 



Feldspar microlites are sometimes thin plates parallel to the clinopina- 

 coid, with the outline formed by basal plane and prism or orthopinacoid. 

 They are often in Carlsbad twins, in juxtaposition and cruciform. These 

 are probably orthoclase. In some less siliceous glasses the feldspar crystals 

 are tabular and rectangular with projecting corners. With these are asso- 

 ciated lath-shaped crystals, forked or fibrous at the ends, which are probably 

 oligoclase. 



Quartz occurs in minute hexagonal pyramids, whose form and double 

 refraction can be distinctly recognized. They range in size from 0.002 to 

 0.015 mm. in diameter. They may easily escape detection, and were over- 

 looked in the obsidian of Obsidian Cliff when the first study of it was 

 made. They are not found in all the rhyolitic glasses in the Yellowstone 



