A chemical sedimentary rock composed of calcium carbonate, formed by evaporation as a surficial, spongy, porous, semifriable incrustation around the mouth of a hot or cold spring or seep, or along a stream carrying calcium carbonate in solution, and exceptionally as a thick, bulbous, concretionary or compact deposit in a lake or along its shore. It may also be precipitated by algae or bacteria. The hard, dense variety is travertine. The term is rarely applied to a similar deposit consisting of silica. It is not to be confused with tuff. Etymol: Italian tufo. Compare: sinter petrified moss.