Calcareous Tufa
a deposition from springs, streams, or underground water, from which it is precipitated partly by the escape of carbonic acid which acts as a solvent, and partly by evaporation of the water. It is usually white, creamy-white, yellowish, brownish in color, but other hues occur, and variegated and mottled varieties are not uncommon. It is of variable texture some or kinds being and consistency; not uncommon. It is of variable texture porous or cellular. These cellular varieties have been deposited from the waters of springs, and often contain vegetable and animal remains, as leaves, twigs, nuts, moss, insects, land and fresh-water shells, etc. The so-called twigs, nuts, moss, insects, land and a good example of the formation of calfresh-water shells, etc. The so-called "petrifying springs" of Matlock afford sition from calcareous waters is on a very extensive scale, as along the river Anio, at Tivoli, near Rome, where calcareous tufa occurs in masses many feet in thickness. In that district the formation is harder and more compact, and under the name of travertino is used as a building stone at Rome. Calcareous tufa is abundantly deposited from thermal springs, as in the Yellowstone region of the United States. The calcareous incrustations so commonly seen in caverns in limestone rocks are varieregion of the United States. The calas stalactites and stalagmites.