was usually applied to floors, where it was at first executed in pebbles or small marble tiles. During the Early Christian period glass tiles were used extensively for wall and vault decorations, the vault of the Mausoleum of Santa Costanza in Rome being a fine example of decorative, non-narrative Christian mosaic. A perhaps unique example of Late Roman pictorial mosaic is the magnificent apsidal mosaic of the Church of Santa Pudenziana. The nearby church, dedicated to her sister Santa Prassede, has mosaics which are Byzantine in style. Mosaic was a favourite form of decoration in the Byzantine period and richly decorated churches in this style can be seen throughout Greece, in Turkey, Italy, Sicily, Russia and other countries. In the 19th century, gold mosaics were applied to the domes of the chancel of St Paul's Cathedral in London, illustrating the creation. In Western Europe, however, it was rare north of the Alps. Stone Sculpture in stone is seemingly the most permanent way of creating images. Because stone is durable to the weather, it is the favoured way of adding figurative decoration to the exteriors of church buildings, either with free-standing statues, figures that form a structural part of the building, or panels of pictorial reliefs. Unfortunately with the pollution and acid rain of the 19th and 20th centuries, much architectural sculpture that had remained reasonably intact for centuries has rapidly deteriorated and become unrecognizable in the last 150 years. On the other hand, much sculpture that is located within church buildings is as fresh as the day it was carved. Because it is often made of the very substance of the building which houses it, narrative stone sculpture is often found internally to be decorating features such as capitals, or as figures located within the apertures of stone screens. The first Christian sculpture took the form of sarcophagi, or stone coffins, modelled on those of non-Christian Romans which were often pictorially decorated. Hence, on Christian sarcophagi there were often small narrative panels, or images of Christ enthroned and surrounded by Saints. In Byzantine Italy, the application of stone reliefs of this nature spread to cathedra (bishop's thrones), ambo (reading lecterns), well heads, baldachin (canopy over altar) and other objects within the church, where it often took on symbolic form such as paired doves drinking from a chalice. Capitals of columns tended to be decorative, rather than narrative. It was in