Argillaceous \Ar`gil*la"ceous\, a. [L. argillaceus, fr. argilla.] Of the nature of clay; consisting of, or containing, argil or clay; clayey. {Argillaceous sandstone} (Geol.), a sandstone containing much clay. {Argillaceous iron ore}, the clay ironstone. {Argillaceous schist} or {state}. See {Argillite}.