Travertine

Travertine, a freshwater limestone, deposited by springs, is said to take its name from Tiburtino or Tivoli (the ancient Tibur), where it accumulates very rapidly. At San Filippo, in Tuscany, it forms a foot thick in four months, a hill 1-1/4 mile long, 1/3 mile broad, and over 250 feet high, being made up of it. It varies in texture, but is generally rather porous, and white or light-coloured. It is, however, largely used as a building material in the Coliseum and elsewhere in Rome, in the Roman Pharos in Dover Castle, and in many Kentish churches. Its formation is probably initiated by the action of living green mosses, Hypnun, etc., removing carbon-dioxide from the water, and thus bringing about the precipitation of calcium carbonate. The springs are, therefore, termed "petrifying" ones. The rock is also known as calcareous tufa or sinter.