410 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



netite grains, and a few needles which are possibly pseudobrookite. In an 

 obsidian from the plateau northeast of Obsidian Cliff, which carries many 

 shrunken hollow spherulites (2215), there are scattered microlites of great 

 beauty. The largest are micrographic intergrowths of quartz and feld- 

 spar which sink to the minutest dimensions; also tabular feldspar, simple 

 and in Carlsbad twins, as well as some rectangular forms with horned 

 corners; pyramidal quartz, one of the larger individuals containing a glass 

 inclusion; hexagonal plates of hematite or ilmenite, opaque, with metallic 

 luster; prisms of augite, some knobbed at the ends, like bones; and occa- 

 sional dark-green prisms of hornblende. 



In the same neighborhood is obsidian with duller luster and a greenish 

 tinge, which is less siliceous and approaches dacite in chemical composition. 

 It consists of compact colorless glass (2164) with a multitude of microlites. 

 Many are augite in prisms, many are feldspar, while some are magnetite. 

 The feldspar microlites are mostly tufted or horned and some have multiple 

 twinning. A very few are in Carlsbad twins. There are some larger 

 microscopic crystals of feldspar and augite, but no microscopic phenocrysts. 

 There are no microlites of quartz and no trichites. Another part of this 

 obsidian is still richer in augite, with smaller feldspar needles and magnetite 

 grains. The microstructure is almost andesitic. 



In some of the compact, microlitic glasses perlitic structure is highly 

 developed, as shown in PI. LI, fig. 4. Its character is too well known to 

 need description in this place. 



FORMS OF GROWTH OF MICROSCOPIC CRYSTALS. 



There is a marked tendency exhibited by the microscopic crystals in 

 rhyolitic magmas to form intergrowths and also compound groups of crys- 

 tals. The well-known graphic intergrowth of orthoclase and quartz is one 

 of the most characteristic. Microscopic examples are of frequent occur- 

 rence and have been described in the paper on Obsidian Cliff.' They 

 can be traced from megascopic groups in which the nature of the compo- 

 nent minerals can be determined to microscopic ones of the minutest 

 dimensions in which only the general form, crystalline structure, and 

 general optical behavior can be recognized. Such groups are seen in thin 

 section to consist of several individuals of feldspar intersecting one another, 



1 Seventh Ann. Rept. U. S. Geol. Survey, 1888, pp. 274-276. 



