GNU Collaborative International Dictionary of English

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1.       
   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
   ☞ The Dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine.
   1913 Webster