There are two important heads of my subject to be shortly discussed before I conclude. One of them refers to roofing; the other to coloured decoration. First, as to roofing. I have already explained how this was executed; let us now consider why the modes which we see were adopted. At S. Front the experiment was tried of covering a nave and transepts with a succession of domes resting on pendentives, and supported on pointed arches spanning the nave. These domes were the only covering of the church, and were visible on the outside as well as on the inside. At Conques, the architect, unable to carry domes on the comparatively delicate piers which were all that were required for the division of a nave from its aisles, contrived a barrel-vault for his nave, the thrust of which was resisted by the half barrel-vault of the triforium; a device not improbably obtained from Byzantine churches: for if we compare the section of S. Sophia with that of the crossing and central dome of Notre-Dame-du-Port, we shall find the semi-domes affording abutments for the great domes in the former, absolutely identical in their section with the half barrel-vault, which forms the abutment on the north and south sides of the central dome of the latter.[60] But it was impossible to obtain any light for a clerestory roofed and supported in this fashion, and one is rather disposed to wonder how it was that so many churches should have been built on the same gloomy scheme. It was, no doubt, because in that part of France wooden roofs were thought to be undesirable, and no other economical way was seen of combining the nave and aisles with what was intended to be an indestructible stone roof. I need hardly say that at the same period, in the north of France, in Normandy, and in England, the nave was seldom, if ever, roofed with anything but timber, and the aisles only were vaulted in stone.