The Petersburg Granite is composed of K-feldspar, quartz, plagioclase with minor amounts of biotite and muscovite. This igneous rock solidified deep in the crust during the late Paleozoic (~300 Ma).

Inclusion of biotite-rich schist enclosed in Petersburg Granite. Biotite-rich schist represents a fragment of older rock that was caught up in the granitic magma when the Petersburg granite intruded. Exposure along James River on the south side of Belle Island, Richmond.