Name:  Rhyolite Porphyry

Geologic Name:  None

Reference:  Cross, C.W., On the occurrence of topaz and garnet in lithophyses of
rhyolite:  Am. Jour. Sci., 3rd Ser., vol. 31, p. 432-438, 1886.

Microscopic characters:  Fresh, unaltered phenocrysts of sanidine and oligoclase.
Carlsbad twinning is prominent in most of the sanidine.  Plagioclase twinning common
in oligoclase.  Quartz generally corroded, sometimes subhedral.  Fine-grained, 
equigranular groundmass of feldspar and some quartz.