In like manner the old conventional forms of decoration, derived from Byzantine carvings and MSS. or from Roman remains, were entirely abandoned, and inspiration in decorative design was sought in the natural flowers and plants of the soil. The intellectual development, no less than the artistic, of this great period of revival was boldly represented in its architecture. The timid forms of traditional construction were soon left behind, and scientific methods were introduced. The clumsy mode of sustaining the central vault by the half vault of the side aisles was superseded by the bold and beautiful form of the flying buttress, loaded with pinnacles where needed to secure stability. This scientific invention enabled the architects to dispense with heavy walls and to bring the whole pressure of the vaults on to points, where they were discharged by the flying buttresses. The side walls were only required as enclosing screens, not as supports, so that there was free scope and every inducement for the expansion of the windows, which rapidly progressed till the whole building became, in striking contrast to the dark and gloomy structures of the monastic regime, an edifice of marvellous lightness and elegance, illuminated from floor to vault with walls of glowing glass. The rapid and extensive development of the Gothic style of the North is one of the most remarkable facts in the history of architecture. Within the century following the first appearance of the style in the pointed vaulting of the abbey church of St Denis, erected under the Abbé Suger in 1144, this style reached its highest point. During that period it found expression in most of the great cathedrals of the North of France, such as Paris, Chartres, Sens, Amiens, Beauvais, &c. This occurred contemporaneously with the long and brilliant reign of Philip Augustus, under whom the royal power became consolidated, and the royal domain extended to an extraordinary degree. As the royal domain extended, its Gothic architecture extended with it, and even passed beyond it, and produced a striking effect on the provinces, such as Provence, not yet absorbed into the kingdom of France. Of this we shall meet with several remarkable examples, as in the cathedrals of Carcassonne and Narbonne, where the designs are pure Northern Gothic, and were furnished by a northern architect. But these and similar structures always strike one as having the appearance of exotics; they are evidently imported plants, not native to the soil. There are also, as we shall see, many other buildings in the South in which some of the features only of the Gothic style are adopted, and which exhibit various attempts to ingraft its details on the native art. But even this is not successful, the buildings having neither the lightness and elegance of the Gothic, nor the massive grandeur of the native style.