MICROLITIC EHYOLITIO GLASSES. 409 



Park which cany microlites. In parts of the obsidian of Obsidian Cliff 

 they occur with feldspar microlites and slightly larger micrographic 

 intergrowths of quartz and feldspar in banded swarms, alternating' layers 

 being nearly free from microlites. It was the presence of the microscopic 

 micrographic intergrowths in association with feldspar microlites which 

 led the present writer to infer the presence of quartz microlites, although 

 quartz had not been observed in such form before that time. 1 If quartz 

 crystallized synchronously in conjunction with feldspar to form such 

 micropraphic microlites, and if the feldspar also crystallized alone, why 

 had not quartz also crystallized by itself at the same time! After a very 

 short search the quartz microlites were found in recognizable individuals. 



In the colored glasses, in some instances, a few dark-colored microlitic 

 prisms and grains occur surrounded by a light-colored zone or "halo," 

 which indicates the concentration of the coloring 1 matter in the microlite. 

 In parts of the pumice at Obsidian Cliff there are microlitic grains, prob- 

 ably augite, each of which is surrounded by a minute sphere of colorless 

 substance, about 0.023 mm. in diameter, with a higher index of refraction 

 than the surrounding glass. In another pumice (1909) the outline of the 

 colorless bodies is seen to be jagged, as though made up of crystal indi- 

 viduals, and in some cases they project beyond the glass Avails of vesicular 

 cavities. They are noticeably doubly refracting, and may be incipient 

 spherulitic growths. 



The various kinds of microlites are not always present in like amounts. 

 In many cases the glass appears to swarm with trichites and microlites of 

 augite, in short prisms and grains. In some cases the trichites seem to be 

 magnetite; in others they are augitic needles. In a compact glass (1941) 

 from the plateau south of Madison Lake, which is colorless in thin section, 

 there are long, curved, opaque trichites which are grouped about thin layers 

 of opaque grains, the trichites themselves in many cases consisting of 

 opaque grains, as though the trichite had been disjointed by shrinking. 

 With these are very few other microlites. 



In some of the obsidian at Obsidian Cliff (2210, 2241, 2242) the micro- 

 lites are. mostly feldspar and quartz with needles of augite and a few mag- 



1 The observation by Dr. Kiich of quartz microlites in the dacitic glasses of Colombia was not 

 known to the writer at the time this investigation was made. W. Reiss und A. Stiibel, Reiseu in Siid- 

 Amerika. Geologische Studien in der Republik Colombia. I. Petrologie. I. Die vulkanischeu Ges- 

 teine, bearbeitet von Richard KUch. Berlin, 1892. 



