The floors were paved with a fine mosaic of marble and green serpentine alternating with slabs of white marble or discs of red porphyry. Tribune, arch, and vault, and sometimes other portions of the walls, were decorated with brilliant mosaics and examples of this work, of the fourth, sixth, ninth, and twelfth centuries, and possibly of the second or third, have happily escaped the ravishing hand of the restorer. In the twelfth century the art of marble working underwent a temporary revival under the influence of a talented family of artists, the Cosmati; and a good deal of their work and that of their school is still to be found in Rome, the carved marble and an inlay of mosaic upon marble being easily recognisable in the decoration of the cloisters of the Lateran and of S. Paul's outside the walls, upon ambones, candelabra, and tombs scattered throughout the churches. The straight architectural lines of the Christian basilicas and their subdued colouring of floor and apse produce a delicate and harmonious effect, but they were erected during a debased building period and were not designed for strength, and only a few have weathered the storms of the middle ages and escaped destruction beneath the tasteless restorations of the Renaissance. The new building epoch born in Rome was to be nourished entirely at the expense of the old. Columns and mouldings were transferred bodily from the nearest basilica to furnish the Christian church, and were there arranged haphazard. Simpler still, walls of ancient bricks were quickly run up between the solid columns of a temple; marble casings were torn off to be used as common building stone; statues, carved cornices, and friezes were thrust into lime-kilns which sprang up all over the city wherever the ancient monuments stood thickest; priceless marbles were ground into fragments for making mosaics or were mixed with cement and made into concrete. When Constantine left Rome to found his new capital the city had already degenerated into a squalid provincial town, and fifty years later Jerome could refer to its gilded squalor and its temples lined with cobweb. Already the seal had been put upon the old order when Gratian in 383 abolished the privileges of the pagan places of worship, and quickly disaster followed upon the heels of destruction. Twice Alaric despoiled the city and carried off priceless booty. Vitiges tore the marble from the mausoleum of Hadrian and destroyed the aqueducts; Genseric dismantled the temple of Jupiter; Robert Guiscard laid waste the Campus Martius and other parts of the city by fire. Sieges, sacks, earthquakes, fires, and inundations succeeded each other until the old level of the city was in places buried 50 feet beneath accumulated ruin and rubbish.