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PART XII. EAEEE BUILDING STONES. CHAPTER I. TRAVERTINE. Travertine, anciently called lapis Tiburtinus, is the principal building stone of ancient and modern Rome. According to the views of Lyell, Bischof, and other observers, this rock has been formed by the precipitation of calcareous matter from water charged with carbonic acid gas ; and the enormous masses accumulated at Tivoli were deposited in an extensive lake, which existed at the close of the period of volcanic activity, during which the lavas and tuffs of the Roman territory were formed. 1 Its manner of formation may still be witnessed at the Lago di Zolfo in the Campagna, and at the cataract of the Anio at Tivoli. The travertine used in Rome is chiefly derived from quarries at Tivoli. It is a straw-coloured, 1 Lyell, Principles of Geol, 10th edit. i. 405. Also Bischof, Chemical Geology, i. 155.