Manhattan Island, in the heart of New York City, seems prima facie an unlikely place for seeing geology. However, nice rock outcrops occur in several city parks, including the largest, Central Park. The bedrock in Manhattan is principally high-grade metamorphic rocks, such as gneisses, schists, and marbles (see geologic map of Manhattan). The rock shown below is a typical mica schist that comes from the late Neoproterozoic Manhattan Schist (unit “Zm” on the geologic map linked to above).
Manhattan Schist (4.6 cm across) from the Precambrian of New York City, USA.