igS COMPARATIVE ARCHITECTURE. ordinary shape was like the Roman, an inch and a half in depth, and they were always laid upon a thick l.ed of mortar, as already mentioned. Moulds were used for the pieces forming cornices, and the shafts of columns when of this material were built of circular bricks. The universal use of brickwork made the Byzantines pay great attention to their mortar, composed of lime, sand, and crushed pottery, tiles or bricks, and it remains as hard as that in the best buildings of Rome. The interiors were beautified by richly colored marble pavements in opus sectile or opus Alexandi'inum (page 119). The use of natural stones in mosaics and inlaid pavements had been abolished, and the art of enamelling had arrived at perfec- tion, all the mosaics which still adorn the domes and apses being of colored glass enamel rendered opaque by oxide of tin, an invention which was introduced in the Early Christian period. The extensive use of rich marbles and mosaics caused a f^at treatment, with an absence of mouldings, cornices and modillions, which were subordinate to the decorative treatment. The simple exteriors of brickwork, with bandings of stone, did not leave the same scope for mouldings as in other styles. Flat splays enriched by incised or low relief ornamentation were introduced, and mosaic and marbles were used, in a broad way, as a complete lining to a rough carcass, architectural lines being replaced by decorative bands in the mosaic, which was worked on rounded angles. One surface melts into another as the mosaic is continued from arch and pendentive upwards to the dome, and the gold of the backgrovmd being carried into the figures, unity of surface was always maintained. Although columns of the richest marbles were taken from old buildings, the importation and sale of newly quarried columns and other decorative materials, such as rare marbles, did not in the least decrease. The Theodosian code in fact encouraged this branch of trade and industry, and the mode of ornamentation by means of colored marbles was carried to a greater extent than ever before. The quarries opened by the Romans continued to be used, and the workmen employed in them were governed by imperial decrees issued specially for their guidance. 3. EXAMPLES. Byzantine examples consist mainly of churches and baptisteries. In the former, although a certain number follow the Basilican type, the majority are founded on the circular and polygonal plans of the Roman and Early Christian periods. SS. Sergius and Bacchus, Constantinople (a.d. 527), erected by Justinian, is nearly square in plan, being a rectangle of
