Tufa
a name given to a light, porous, calcareous stone, sometimes having the aspect of a sandstone, at others earthy and inclosing the decomposed remains of vegetable substances; composition, a carbonate of lime; deposited by springs, rivers, and heated waters which have traversed calcareous rocks; sometimes confounded with tuff which is the term specifically applied to a fragmentary deposit sit of a volcanic nature of composed heterogeneous materials.
TULIP