The Collaborative International Dictionary of English v.0.48:

Neptunian \Nep*tu"ni*an\, prop. a. [L. Neptunius belonging to
   Neptune: cf. F. neptunien.]
   1. Of or pertaining to the ocean or sea.
      [1913 Webster]

   2. (Geol.) Formed by water or aqueous solution; as, Neptunian
      rocks.
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   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.
      [1913 Webster] Neptunian