What does argillaceous mean?
The definition of argillaceous
Ar`gil*la”ceous, a.
Etym: [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.