An intrusive igneous rock similar to granite, but contains more plagioclase than potassium feldspar. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.
-- Excerpts from:
USGS/NPS Geology in the Parks Website, 2003, and U.S. National Park Service Website, Yosemite National Park, 2003