cold mineral springs, the rapid precipitation of minerals results from the reduction of acidity when the CO2 gas bubbles out. (These mineral deposits can also be found in dried lakebeds.) Spectacular formations, including terraces, stalactites, stalagmites and 'frozen waterfalls' can result (see, for example, Mammoth Hot Springs). One light-colored porous calcite of this type is known as travertine and has been used extensively in Italy and elsewhere as building material. Travertine can have a white, tan, or cream-colored appearance and often has a fibrous or concentric 'grain'. Another type of spring water deposit, containing siliceous as well as calcareous minerals, is known as tufa. Tufa is similar to travertine but is even softer and more porous. Chaybeate springs may deposit iron compounds such as limonite. Some such deposits were large enough to be mined as iron ore. See also List of hot springs Sweet springs, those with no detectable sulfur or salt content References Cohen, Stan (Revised 1981 edition), Springs of the Virginias: A Pictorial History, Charleston, West Virginia: Quarrier Press. Springs (hydrology) Drinking water Spa waters Water chemistry Bathing Natural environment based therapies Geomorphology