Name:   Andesite Breccia

Geologic Name:  San Juan andesite breccia

Reference: Cross, C.W., and How, Ernest, Description of the Ouray
quadrangle: U.S. Geol. Survey Geol. Atlas, Silverton folio (No. 120),
1906.

Microscopic characters:  The slide shows a porphyritic texture with large
phenocrysts of feldspar largely changed to calcite embedded in a fine
groundmass which shows aggregate polarization.  Albite twinning can be
noted on some of the feldspars.  A few quartz grains and greatly altered
biotite crystals also occur as phenocrysts.  Magnetite, ilmenite coated with
leucoxene, and small apatite crystals are quite abundant.  Faintly pleochroic
chlorite is very abundant in the groundmass.