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trass

(Science: geology) 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.

The dutch trass is made by burning and grinding a soft grayish rock found on the lower Rhine.

Origin: D. Tras or Gr. Trass, probably fr. It. Terrazzo terrace. See Terrace.