No marble quarries exist in the vicinity of Rome.  The Sabine Hills are indeed of limestone formation, and large masses of travertine, a fresh-water limestone of igneous origin, occur here and there, but no mineral approaching marble in texture and appearance is found within a very considerable radius of the city.  The nearest source of supply is at Cesi, near the celebrated “Falls of Terni,” about forty-five miles from Rome, where “Cotanella,” the red marble of the Roman States, is found, of which the great columns supporting the arches of the side aisles of St. Peter’s are formed.  The hills and rocks of Rome are all volcanic, and only the different varieties of eruptive rock were first employed for building purposes.  The oldest monuments of the kingly period, such as the Cloaca Maxima, the Mamertine Prison, the Walls of Servius Tullius, and some of the earliest substructures on the Palatine Hill, were all built of the brown volcanic tufa found on or near their sites.  This is the material of which the famous Tarpeian Rock and the lower part of most of the Seven Hills is composed.  It is the oldest of the igneous deposits of Rome, and seems to have been formed by a conglomerate of ashes and fragments of pumice ejected from submarine volcanoes whose craters have been completely obliterated.  It reposes upon marine tertiary deposits, and over it, near the Church of Sta.  Agnese, where it is still quarried for building stone, rests a quaternary deposit, in which numerous remains of primeval elephants have been found.  Though the Consular or Republican period was a very stormy one, and the reconstruction of the city, after its partial demolition by the Gauls, seems to have been too hurried to allow much attention to be paid to the materials and designs of architecture, yet there are numerous indications in the existing remains of that period that there was a decided advance in these respects upon the ruder art of a former age.  Finer and more ornamental varieties of volcanic stone were introduced from a distance, such as the peperino or grayish-green tufa of the Alban Hills, the Lapis Albanus of the ancients, with its glittering particles of mica interspersed throughout its mass; the hard basaltine lava from a quarry near the tomb of Caecilia Metella, on the Appian Way, and from the bed of the Lago della Colonna, once the celebrated Lake Regillus, to which the name of Lapis Tusculanus or Selce was given; and the Lapis Gabinus or Sperone, a compact volcanic concrete found in the neighbourhood of the ancient Gabii on the road to Tivoli, extensively used in the construction of the earliest monuments, particularly the Tabularium and the huge Arco de Pantani.  Brick was also largely employed in the construction of the foundations and inner walls of public buildings, being arranged at a later date into ornamental patterns, to which the names of opus incertum and opus reticulatum were given; and in the manufacture