Within the walls of the city interment was very properly forbidden by the law, and at a convenient distance beyond its crowded precincts, large plots of ground could hardly be obtained; but the formation of subterranean cemeteries on a vast scale was greatly facilitated by the geological formation of the land. Three different kinds of stone compose the groundwork of the Roman campagna: the _tufa litoide_, as hard and durable as granite, which furnished the materials for the palaces and temples of the Cæsarian city; the _tufa granolare_, which, though consistent enough to retain the form given it by the excavators, cannot be hewn or extracted in blocks, and the loose _tufa friabile_, or _pozzuolana_, which has been extensively used from the earliest ages for mortar or Roman cement. It is evident that neither the hard lithoid nor the loose tuff were suitable for the excavation of the catacombs, while this purpose could be admirably attained in the courses of the granular tuff, which, though not too hard to be worked, is yet solid enough to make walls for long and intricate passages, to be hewn into arches vaulting over deep recesses for the reception of coffins, and to support floor below floor down to the utmost depth to which the formation reaches. Neither in the stone quarries nor in the sand-pits of ancient Rome is there the slightest sign that they were ever used for the purposes of sepulture, while in the granular tuff not a yard seems to have been excavated except for the making of tombs, which line the walls throughout their prodigious length, as close to one another as the berths in the sides of a ship. Though the most ancient catacombs were excavated by the Jews, yet these excavations are of a very limited extent when compared with those of Christian origin, where, instead of the seven-branched candlestick and other sacred emblems of the Jewish persuasion, every ornament or inscription marked or painted upon the walls bears witness to the faith of those who were deposited—to use the peculiar and appropriate expression—within these narrow cells. Everywhere we see Christian symbols only—the horse, emblematic of strength in the faith; the hunted hare, of persecution; the dove and the cock, of the Christian virtues of vigilance and meekness; the peacock and the phœnix, of resurrection; the anchor, of hope in immortality; the palm-leaf, of the martyr’s triumph over death, and many others.