302 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



The pyroxene-andesite occurring as a massive lava flow west of the 

 head of Mink Creek lias a habit somewhat different from that of the 

 pyroxene-andesites already described. It has a smaller number of pheno- 

 crysts, which are labradorite, distinctly pleochroic hypersthene, and augite, 

 the pyroxenes being more prismatic than usual. The groundmass when 

 very dense is globulitic and microlitic glass, which is dark colored where 

 the microlites are very minute. Through it are scattered long prismatic 

 microlites of hypersthene, augite, and feldspar. In varieties with larger 

 microlites the color is lighter, and it is seen that pyroxene is more abundant 

 than feldspar ; magnetite also is abundant. Apatite occurs in comparatively 

 few large crystals, with brownish color and distinct pleochroism. Tridy- 

 mite is present in small aggregates. 



Basaltic andesite and basalt. — These rocks are like the pyroxene-andesites in 

 habit, both megascopically and microscopically. They consist of an 

 aphanitic groundmass with multitudes of small phenocrysts of feldspar 

 and pyroxene and more or less altered olivine. In some varieties the 

 phenocrysts are almost absent, or are of microscopic proportions. The 

 colors of the rocks are darker as a whole than those of most of the andesites. 

 Dark grays to black and red are most common. Light grays occur. Many 

 of the rocks are porous or finely vesicixlar. 



In thin sections the groundmasses are seen to be glassy, with abundant 

 microlites of feldspar, pyroxene, and magnetite, the latter minerals being 

 rather more abundant than in the pyroxene-andesites, and being equal to, 

 or sometimes more numerous than, the feldspars. The microlitic pyroxenes 

 appear to be augite. The feldspars are lime-soda feldspars, apparently 

 about andesine-labradorite, but not always of the same kind. The glass is 

 usually brown globulitic, when in noticeable amount. Holocrystalline 

 modifications occur; they are very fine grained and are formed by the 

 growing' together of microlites. Groundmasses so crowded with iron oxide 

 as to be opaque, even in very thin sections, are common. 



The feldspar phenocrysts are labradorite-bytownite, in some rocks 

 being more calcic than the proportion An 3 Ab 2 , in other cases having the 

 optical properties of this variety of labradorite. The forms are mostly 

 rectangular and prismatic, tabular forms being common in some rocks. The 

 twinning is according to albite, pericline, and Carlsbad laws, usually all three 

 combined; occasionally only two. Zonal structure is pronounced, especially 



