The Collaborative International Dictionary of English v.0.48:

Metasomatism \Met`a*so"ma*tism\, n. [Pref. meta- + Gr. ?, ?,
   body.] (Geol.)
   An alteration in a mineral or rock mass when involving a
   chemical change of the substance, as of chrysolite to
   serpentine; -- opposed to ordinary metamorphism, as implying
   simply a recrystallization. -- Met`a*so*mat"ic, a.
   [1913 Webster]