Source : Webster's Revised Unabridged Dictionary (1913)

Argillite \Ar"gil*lite\, n. [Gr. ? clay + -lite.] (Min.)
   Argillaceous schist or slate; clay slate. Its colors is
   bluish or blackish gray, sometimes greenish gray, brownish
   red, etc. -- {Ar`gil*lit"ic}, a.

Source : WordNet®

argillite
     n : a sedimentary rock differing from shale in being bound by
         silica and from slate in having no slate cleavages