Deposits of calcareous tufa have formed and are probably forming now ill numerous alkaline-carbonate lakes of the western United States. Many of these deposits in the Mono and Lake Lahontan basins are mushroom-shaped and lie at the orifices of what are or were sublacustrine springs. The mushroom-shaped masses are composed of concentrically arranged lithoid, dendritic, and thinolitic tufa with the core usually lithoid and the dendritic, and thinolitic layers alternating concentrically outward. The tufa is shown here to be formed as the result of mixing waters of varying calcium and carbonate content which react essentially in accordance with the solubility product of calcium carbonate in distilled water. The thinolitic and dendritic tufas are believed to be the result of either solution and redeposition within the masses or of varying rates of deposition on the outside of the tower. The possibility is presented that many of the deposits of lithoid tufa on the shores of alkaline lakes are formed by precipitation at the contact of rain-wash and lake water.