Source : Webster's Revised Unabridged Dictionary (1913)

Neptunian \Nep*tu"ni*an\, a. [L. Neptunius belonging to Neptune:
   cf. F. neptunien.]
   1. Of or pertaining to the ocean or sea.

   2. (Geol.) Formed by water or aqueous solution; as, Neptunian
      rocks.

   {Neptunian races} (Ethnol.), the Malay and Polynesian races.
      

   {Neptunian theory} (Geol.), the theory of Werner, which
      referred the formation of all rocks and strata to the
      agency of water; -- opposed to the Plutonic theory.