The white marbles hitherto described were principally for exterior use. But as Roman wealth and luxury increased coloured marbles were employed for internal decoration; and the effects which the Greeks obtained by the application of pigments, the Romans obtained by the rich hues of precious marbles incrusting their buildings, and durable as these buildings themselves. At first these rare materials were used with a degree of moderation, chiefly in the form of mosaics of small discs or cubes for the pavements of halls and courts. But at length massive pillars were constructed of them, and the vast inside brick surfaces of imperial baths and palaces were crusted over and concealed by slabs of rare and splendid marbles, the lines of which had no necessary connection with the mass behind or beneath. Carthage from the spoils of its temples supplied Rome with many of its rarest columns; and it is probable that not a few of these survive in the Christian basilicas that occupy the sites and were built out of the materials of the old Pagan structures. With the decay of the Roman Empire the use of coloured marbles in art increased, so that even busts and statues had their faces and necks cut in white and the drapery in coloured marble. It attained its fullest development in the Byzantine style, of which, as it appeals to the senses more by colour than by form, it is a predominant characteristic, necessary to its vitality and expression. The early Christian builders contemplated this mode of decoration for their interiors only. Very rarely had they the means to apply it to the outside surface, as in St. Mark's in Venice, which is the great type of the Byzantine church, coloured within and without with the rich hues of marbles and mosaics. Our great Gothic cathedrals, as an eminent architect has said, were the creation of one thought, and hence they were complete when the workmen of the architects left them, and their whole effect is dominated by one idea or one set of ideas; but the early Roman churches were the results of a general co-operation of associated art, and the large and plain surfaces of the interiors were regarded by the sculptor as a framework for the exhibition of his decorative art. Colour was lavished in veneers of rare marbles, and costly mosaics and frescoes covering the walls. There was thus "less unity of purely architectural design, but a greater amount of general artistic wealth."