roofed with a ribbed vault. It is but a variant of the standard vaulting types described in this chapter. CHAPTER II TRANSEPT AND CROSSING VAULTS TRANSEPTS WITH TUNNEL VAULTS Because of the close resemblance in plan and structure between them, the transept was vaulted like the nave in by far the larger number of instances. Thus in the Romanesque schools, where the nave was tunnel vaulted, similar vaults were generally placed above the transept as well. They were, moreover, well suited to this position, especially where there were no transept aisles, for the outer walls running down to the ground afforded them excellent support and also provided space for windows of considerable size. It is not surprising, therefore, to find the tunnel-vaulted transept the standard in Romanesque church architecture, an example appearing even as far north as Jedburgh Abbey in Scotland, although this was a church of the Norman school in which the nave was probably originally roofed with wood.[282] Even in the school of Perigord, where the naves are domed, the transept is frequently covered with a tunnel vault as, for example, in the churches of Souillac (Lot), Tremolac (Dordogne), and Vieux Mareuil (Dordogne), in the cathedral of Angoulême, and perhaps originally in the cathedral of Saint Front at Périgueux.[283] Occasionally, however, other forms displace the tunnel vault in transept construction. One of these appears in the abbey church of Cluny (Saône-et-Loire) (early twelfth century). Here the bays of the transept, corresponding to the side aisles of the church are tunnel vaulted, but beyond these, there are two projecting bays, the inner one square and covered by an octagonal dome on trumpet squinches, the outer covered with a tunnel vault at a lower level[284] than that over the two bays adjoining the crossing. Above the dome rises an octagonal tower and spire, and the whole composition of this bay shows that it was intended to be a flanking tower like those to be seen at Angoulême, Tréguier (Côtes-du-Nord), and Exeter cathedrals. For such a tower, a dome is more suitable than a tunnel vault, because it exerts less outward thrust. This is also better distributed. [Illustration: FIG. 49.--CLERMONT-FERRAND, NOTRE DAME-DU-PORT.]