Name:  Rhyolite

Geologic Name:  None

Reference:  Richardson, G.B., Description of the Castle Rock quadrangle, Colorado:
U.S. Geol. Survey, Geol. Atlas, Castle Rock folio (no. 198), 1915

Microscopic characters:  Porphyritic texture.  The phenocrysts are quartz, sanidine,
sodic plagioclase, biotite and magnetite which is more or less completely altered
to hematite.  The groundmass is microcrystalline-granular, and slightly clouded by
iron oxide.  Numerous drawn-out air cells indicate flow structure.  There are a few
crystals of rutile.