Name:  Aporhyolite (Felsite)

Geologic Name:  Mattapan felsite

Microscopic characters:  Porphyritic texture.  The groundmass generally shows
aggregate polarization effects.  Minute scales of mica are disseminated
throughout it.  In places the groundmass is more coarsely crystalline and is
composed of intergrowths of quartz and feldspar.  Large phenocrysts of quartz,
orthoclase and plagioclase are present in this part.  There are a few crystals
of magnetite and much leucoxene in irregular smears.  Secondary calcite is
quite abundant.  Abundant ferric oxide stain outlines the flow structure best
when viewed in the reflected light.