Christian architecture in Syria soon, however, diverged from Roman traditions. The abundance of hard stone, the total lack of clay or brick, the remoteness from Rome, led to a peculiar independence and originality in the forms and details of the ecclesiastical as well as of the domestic architecture of central Syria. These innovations upon Roman models resulted in the development of distinct types which, but for the arrest of progress by the Mohammedan conquest in the seventh century, would doubtless have inaugurated a new and independent style of architecture. Piers of masonry came to replace the classic column, as at Tafkha (third or fourth century), Rouheiha and Kalb Louzeh (fifth century? Fig. 69); the ceilings in the smaller churches were often formed with stone slabs; the apse was at first confined within the main rectangle of the plan, and was sometimes square. The exterior assumed a striking and picturesque variety of forms by means of turrets, porches, and gables. Singularly enough, vaulting hardly appears at all, though the arch is used with fine effect. Conventional and monastic groups of buildings appear early in Syria, and that of +St. Simeon Stylites+ at Kelat Seman is an impressive and interesting monument. Four three-aisled wings form the arms of a cross, meeting in a central octagonal open court, in the midst of which stood the column of the saint. The eastern arm of the cross forms a complete basilica of itself, and the whole cross measures 330 × 300 feet. Chapels, cloisters, and cells adjoin the main edifice. [Illustration: FIG. 70.--CATHEDRAL AT BOZRAH.] Circular and polygonal plans appear in a number of Syrian examples of the early sixth century. Their most striking feature is the inscribing of the circle or polygon in a square which forms the exterior outline, and the use of four niches to fill out the corners. This occurs at Kelat Seman in a small double church, perhaps the tomb and chapel of a martyr; in the cathedral at +Bozrah+ (Fig. 70), and in the small domical church of +St. George+ at +Ezra+. These were probably the prototypes of many Byzantine churches like St. Sergius at Constantinople, and San Vitale at Ravenna (Fig. 74), though the exact dates of the Syrian churches are not known. The one at Ezra is the only one of the three which has a dome, the others having been roofed with wood. The interesting domestic architecture of this period is preserved in whole towns and villages in the Hauran, which, deserted at the Arab conquest, have never been reoccupied and remain almost intact but for the decay of their wooden roofs. They are marked by dignity and simplicity of design, and by the same picturesque massing of gables and roofs and porches which has already been remarked of the churches. The arches are broad, the columns rather heavy, the mouldings few and simple, and the scanty carving vigorous and effective, often strongly Byzantine in type.