or TARRAS, in Mineralogy, a species of argillaceous earth, differing little from puzzolana, but in being more compact and hard, porous and spongy. It is generally of a whitish yellow colour, and contains more heterogeneous particles, as spar, quartz, shoerl, &c. and something more 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 slaked lime, and one of sand, forms a cement that answers extremely well for buildings in water, for it soon hardens and becomes like a stone.