A survey of Sasanian buildings leads to the conclusion that a singular want of technical skill was displayed in their vaulting system. The vault and the dome may have been born in Mesopotamia, but they lingered there in a state of immaturity. The barrel vault, the vault on transverse arches, the dome on Persian squinches, or in smaller dimensions on the horizontal bracket, these were the only forms which were employed. If an inclined plane was to be covered, the barrel vault was split up into sections and raised in steps; if the barrel vaults met at right angles, they were carefully separated from one another. At Ukhaiḍir the groined vault is added to this slender stock of forms, but it is not used in many places where it might be expected to appear, and when it is employed, it is only with the utmost precaution. As far as the invention shown in the Mesopotamian regions is concerned, we might to-day be obliged to content ourselves with the barrel vault and the dome poised carefully upon four walls (or little better); but the Greek builders of the Mediterranean coast-lands stepped into the breach, and it is primarily to them that we owe the development of the elementary principles of oriental vaulting. I have already alluded to a series of early Mohammadan buildings which are of the utmost importance to the study of Ukhaiḍir, the Umayyad ḥîrahs which stand upon the frontiers of Syria. On the western side of the desert the authority of the khalifs had been preceded by the authority of Imperial Rome. Lands which were occupied by Roman armies were endowed with a solid heritage, more enduring than any political domination has proved to be. To this day the traveller to Petra has the paved Roman road under his feet for many a mile; he can reckon his journey by Roman milestones, and daily he will pass by shattered wall and piles of ruin which mark the site of Roman watch-tower and Roman fortified camp. After the lapse of eighteen hundred years these massive structures still offer a meagre shelter to the Beduin shepherds and their flocks, and in the seventh century, when the Umayyad khalifs fled from their cities to the beautiful solitudes of the Syrian desert, most of the castles of the Roman limes, which had been re-occupied by the Ghassânid allies of Byzantium, were standing in all their towered strength. Here indeed was an inheritance for those who loved the wilderness; where the Roman legionaries had languished in interminable exile, the children of the desert held their court.