ANDESITES OF ABSAEOKA EANGE. 293 



mass is crystallized. The more glassy varieties afford the best opportunity 

 for observing the character of the microlites. For this reason it may be best 

 to begin with the description of the most glassy varieties. Of these, some 

 have glass, which is colorless in thin section, with many distinctly crystal- 

 lized microlites, which, however, are not crowded together. The microlites 

 are feldspar, pyroxene, and magnetite. The feldspars have not always the 

 same habit. In some varieties of the rock the microscopic feldspars are 

 rectangular, square, and elongated, with lamellar twinning, both albitic 

 and periclinic, the symmetrical extinction angles of 33° to 35° indicating 

 anorthite. They range in size from the most minute crystals to those 0.6 mm. 

 long. They are at times indented at the ends. In other varieties of the 

 rock the microlites of feldspar are partly thin tabular crystals, sometimes 

 crossing one another in Carlsbad twinning position. They are lime-soda 

 feldspars yielding lath-shaped cross section. In the glassiest rocks the feld- 

 spars have the most abundant glass inclusions, which occur even in micro- 

 scopic crystals, although the smallest feldspars are quite free from inclusions. 



The pyroxene microlites, like those of feldspar, range in size from the 

 minutest to those that might be called small phenocrysts. Augite and 

 hypersthene are both present, but in some cases hypersthene is the prevail- 

 ing species. Both pyroxenes occur in prisms, sometimes stout, sometimes 

 slender, occasionally very thin. They are often cracked across' as with 

 basal parting, the pieces of crystal slightly separated in some cases. These 

 grade into cases in which the pyroxene prisms are represented by a line of 

 grains whose distance apart equals or exceeds the thickness of the grain. 

 From the resemblance of these grains to the globulites in some of the 

 glassy groundmasses it is probable that these globulites are augite to a con- 

 siderable extent. 



Magnetite crystals are frequently inclosed in the pyroxene in great 

 numbers, but in some cases they are nearly free from them. The mag- 

 netite crystals are idiomoi'phic in many cases, but in others their forms are 

 indistinct. One perfectly developed crystal which was inclosed in a feldspar 

 had the form of a dodecahedron combined with an octahedron. Apatite 

 occurs in minute hexagonal prisms. 



In sToundmasses where the <dass is crowded with microlites their 

 forms appear to be the same, but on account of their frequent superposition 

 the forms are not so easily observed. 



