The Collaborative International Dictionary of English v.0.48:

Terras \Ter"ras\, n. (Min.)
   See ?rass.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Trass \Trass\, n. [D. tras or Gr. trass, probably fr. It.
   terrazzo terrace. See Terrace.] (Geol.)
   A white to gray volcanic tufa, formed of decomposed trachytic
   cinders; -- sometimes used as a cement. Hence, a coarse sort
   of plaster or mortar, durable in water, and used to line
   cisterns and other reservoirs of water. [Formerly written
   also tarras, tarrace, terras.]
   [1913 Webster]

   Note: The Dutch trass is made by burning and grinding a soft
         grayish rock found on the lower Rhine.
         [1913 Webster]