tuf

A stone or porous substance formed by depositions from springs or rivulets, containing much earthy matter in solution. Tufa is also formed by the concretion of loose volcanic dust or cinders, cemented by water, or by the consolidation of mud thrown out of volcanoes. The disintegration and subsequent consolidation of basaltic rocks, forms a kind of tufa, called by the German geologists, trap-tuff.