Webster's Revised Unabridged Dictionary · 1913

Neptunian

Neptunian , proper adjective

[Latin Neptunius belonging to Neptune: compare French neptunien.]

1.
Of or pertaining to the ocean or sea.
2.
(Geology) Formed by water or aqueous solution; as, Neptunian rocks.
Collocations (2)
Neptunian races (Ethnology) , the Malay and Polynesian races.
Neptunian theory (Geology) , the theory of Werner, which referred the formation of all rocks and strata to the agency of water; -- opposed to the Plutonic theory.

Neptunian , proper noun

[Compare French neptinien, neptuniste.]

(Geology) One who adopts the Neptunian theory.

Also: Neptunist