Source : Webster's Revised Unabridged Dictionary (1913)

Greensand \Green"sand`\ (-s?nd`), n. (Geol.)
   A variety of sandstone, usually imperfectly consolidated,
   consisting largely of glauconite, a silicate of iron and
   potash of a green color, mixed with sand and a trace of
   phosphate of lime.

   Note: [hand]Greensand is often called {marl}, because it is a
         useful fertilizer. The greensand beds of the American
         Cretaceous belong mostly to the Upper Cretaceous.

Source : WordNet®

greensand
     n : an olive-green sandstone containing glauconite