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