Name:  Biotite Gneiss

Geologic Name:  Northbridge Gneiss

Reference:  Emerson, B.K., Geology of Massachusetts and Rhode Island:
U.S. Geol. Survey, Bull. 597, p. 155-156, 1917.

Microscopic characters:  Schistose texture.  The cheif constituents are
quartz, plagioclase, biotite and dark green hornblende.  Epidote
grains and zircon crystals and grains are very abundant.  Garnet is
also present.