(except its portico and internal columns), the temples of Peace, of Venus and Rome, and of Minerva Medica, were of brick. So were the great baths of Titus, Caracalla, and Diocletian, the villa of Hadrian, the city walls, the villa of Mecaenas at Tivoli, and most of the palaces of the nobility,--although, like many of the temples, they were faced with stone. The Colosseum was of travertine, a cheap white limestone, and faced with marble. It was another custom to stucco the surface of brick walls, as favorable to decorations. In consequence of the invention of the arch, the Romans erected a greater variety of fine structures than either the Greeks or Egyptians, whose public edifices were chiefly confined to temples. The arch entered into almost every structure, public or private, and superseded the use of long stone-beams, which were necessary in the Grecian temples, as also of wooden timbers, in the use of which the Romans were not skilled, and which do not really pertain to architecture: an imposing edifice must always be constructed of stone or brick. The arch also enabled the Romans to economize in the use of costly marbles, of which they were very fond, as well as of other stones. Some of the finest columns were made of Egyptian granite, very highly polished. The extensive application of the arch doubtless led to the deterioration of the Grecian architecture, since it blended columns with arcades, and thus impaired the harmony which so peculiarly marked the temples of Athens and Corinth; and as taste became vitiated with the decline of the empire, monstrous combinations took place, which were a great fall from the simplicity of the Parthenon and the interior of the Pantheon. But whatever defects marked the age of Diocletian and Constantine, it can never be questioned that the Romans carried architecture to a perfection rarely attained in our times. They may not have equalled the severe simplicity of their teachers the Greeks, but they surpassed them in the richness of their decorations, and in all buildings designed for utility, especially in private houses and baths and theatres. The Romans do not seem to have used other than semicircular arches. The Gothic, or Pointed, or Christian architecture, as it has been variously called, was the creation of the Middle Ages, and arose almost simultaneously in Europe after the first Crusade, so that it would seem to be of Eastern origin. But it was a graft on the old Roman arch, in the curve of the ellipse rather than the circle.