or Trass, in mineralogy, a species of argillaceous earth. It differs but little in its principles from puzzolana, but is much more compact and hard, porous and spungy. It is generally of a whitish yellow colour, and contains more heterogeneous particles, as spar, quartz, spherulites, &c., and something more of calcareous earth; it effervesces with acids, is magnetic, and fusible per se. When pulverized, it serves as a cement, like puzzolana. It is found in Germany and Sweden.

A species of red earth has been found in the parish of St Elizabeth in Jamaica, which turns out to be an excellent substitute for terras or puzzolana earth, and may therefore be of great value to the inhabitants of the West Indies.

One measure of this earth, mixed with two of well slackened lime and one of sand, form a cement that answers extremely well for building any dam or bridge, or any structure in water, for it will soon harden and become like a stone.