The Collaborative International Dictionary of English v.0.48:

Argillaceous \Ar`gil*la"ceous\, a. [L. argillaceus, fr.
   argilla.]
   Of the nature of clay; consisting of, or containing, argil or
   clay; clayey.
   [1913 Webster]

   Argillaceous sandstone (Geol.), a sandstone containing much
      clay.

   Argillaceous iron ore, the clay ironstone.

   Argillaceous schist or state. See Argillite.
      [1913 Webster]

WordNet (r) 3.0 (2006):

argillaceous
    adj 1: resembling or containing clay; "argillaceous rocks" [syn:
           argillaceous, clayey] [ant: arenaceous, sandlike,
           sandy]