The Collaborative International Dictionary of English v.0.48:

Lias \Li"as\ (l[imac]"as), n. [Cf. F. lias, fr. liais sort of
   limestone, OF. also liois; perh. of Celtic origin, cf. Armor.
   liach, leach, a stone, Gael. leac, W. llech. Cf. Cromlech.]
   (Geol.)
   The lowest of the three divisions of the Jurassic period; a
   name given in England and Europe to a series of marine
   limestones underlying the O["o]lite. See the Chart of
   Geology.
   [1913 Webster]