Greywacke or Graywacke -LRB- MISC@1 grauwacke , signifying a grey , earthy rock -RRB- is a variety of sandstone generally characterized by its hardness , dark color , and poorly-sorted , angular grains of quartz , feldspar , and small rock fragments or lithic fragments set in a compact , clay-fine matrix .

Greywacke -LRB- MISC@1 grauwacke , signifying a grey , earthy rock -RRB- is a type of sandstone .
