The peculiarly English feature of an open roof is hardly ever met with in any shape: yet though stone vaults are almost universal, they are rarely equal in scientific skill to the best of those in our own country. In transitional examples, many very singular instances of the expedients employed before the pointed vault was fully developed can be found. In some of the central and southern districts, domes, or at least domical vaults, were employed. (See the section of Fontevrault, Fig. 31). The dome came in from Byzantium. It was introduced in Perigord, where the very curious and remarkable church of St. Front (begun early in the eleventh century) was built. This is to all intents a Byzantine church. It is an almost exact copy in plan and construction of St. Mark's at Venice, a church designed and built by Eastern architects, and it is roofed by a series of domes, a peculiarity which is as distinctive of Byzantine (_i.e._, Eastern early Christian), as the vaulted roof is of Romanesque (or Western early Christian) architecture. Artists from Constantinople itself probably visited France, and from this centre a not inconsiderable influence extended itself in various directions, and led to the use of many Byzantine features both of design and ornament. As features in the exterior of their buildings, the roofs have been in every period valued by the French architects; they are almost always steep, striking, and ornamented. All appropriate modes of giving prominence and adding ornament to a roof have been very fully developed in French Gothic architecture, and the roofs of semicircular and circular apses, staircase towers, &c., may be almost looked upon as typical.[23] _Openings._ The treatment of openings gives occasion for one of the most strongly marked points of contrast between French and English Gothic architecture. With us the great windows are unquestionably the prominent features, but with the French the doors are most elaborated. This result is reached not so much by any lowering of the quality of the treatment bestowed upon the windows, but by the greatly increased importance given to doorways. The great portals of Notre Dame at Paris (Fig. 33), Rheims, or Amiens (Fig. 35), and the grand porches of Chartres may be named as the finest examples, and are probably the most magnificent single features which Gothic Art produced in any age or any country; but in its degree the western portal of every great church is usually an object upon which the best resources of the architect have been freely lavished. The wall is built very thick so that enormous jambs, carrying a vast moulded arch, can be employed. The head of the door is filled with sculpture, which is also lavishly used in the sides and arch, and over the whole rises an ornamental gable, frequently profusely adorned with tracery and sculpture, its sides being richly decorated by crockets or similar ornaments, and crowned by a sculptured terminal or finial.