After the first century B.C. concrete became a favourite building material. The walls so made were lined with stucco and faced without in various fashions, the variety of the facing determining with considerable accuracy the date of the fabric. The earliest facing, of the first and second century B.C., was of irregular blocks of tufa set in cement, and is known as the _opus incertum_. This was replaced in the middle of the first century B.C. by tufa blocks cut in squares and set diagonally giving the appearance of a network and hence known as _opus reticulatum_. In or after the first century A.D. this fashion was superseded by a facing of triangular bricks set point inwards, and by the end of the third century bricks were mixed with the _opus reticulatum_, a style known as _opus mixtum_. To the casual observer the narrow brown bricks of the ruined buildings of ancient Rome seem to play an important part, but, with few exceptions, they are merely a brick facing upon concrete. Up to the first century B.C. there was little or no splendour or decoration introduced into the buildings of Rome, and the city of Augustus' inheritance was a city of sober-hued, volcanic rock. When marble was first sparingly used, Livy reprobates it as too showy and extravagant. Notwithstanding, the fashion rapidly spread, first in the embellishment of public buildings, then for private houses as well until in the first century of the Empire it became a common building stone. For nearly three centuries it was imported into the city in a continuous flow from the quarries of Greece and Egypt. The native Luna marble, the modern Carrara, was not at first worked, but thousands of slaves and convicts toiled in the quarries of the Roman provinces. The great blocks were numbered and stamped with the name of the reigning emperor and shipped off in the great triremes across the Mediterranean to Ostia. Thence the trading vessels were towed by oxen up the river to Rome, their slow progress ceasing with nightfall, when they were drawn up and moored to the banks till next morning, bands of _vigiles_ watching over the safety of their cargoes and restraining their lawless crews from acts of brigandage. At their journey's end, the cargoes were unloaded upon the marble wharf beneath the Aventine; here unused blocks still lie upon the site of the once busy _Marmoratum_, now a deserted quay beside a deserted river; and the harbour of Ostia, built by King Ancus Martius at the river's mouth, is now four miles inland.