The Collaborative International Dictionary of English v.0.48:

Marlstone \Marl"stone`\, n. (Geol.)
   A sandy calcareous straum, containing, or impregnated with,
   iron, and lying between the upper and lower Lias of England.
   [1913 Webster]

WordNet (r) 3.0 (2006):

marlstone
    n 1: metamorphic rock with approximately the same composition as
         marl [syn: marlite, marlstone]