In most of the collegiate churches the barrel vaults supported a roof composed of carefully wrought flag-stones. These stones are arranged in courses, running from the eaves to the ridge, and every alternate course is higher than, and rests on, the edges of the intermediate courses. Each stone also overlaps the course which is below it in the slope of the roof. There is thus a considerable amount of cutting and fitting required, which is usually carefully executed. Sometimes each stone is hollowed in the centre, so as to carry the water away from the joints. The gutters are also wrought in stone on the same principle. Roofs of this description might evidently be made almost level, and in the case of many of the castles (as on the keep of Craigmillar Castle) that is done, and a platform for defence is thus created. In the churches, however, the stone roofs are usually pretty steep. It is remarkable that this form of roof was a reproduction in Scotland, in the fifteenth century, of a fireproof form of construction which was much used in Provence in the eleventh and twelfth centuries. But in this country it was to all appearance an independent invention, as Provence in the fifteenth century was, architecturally speaking, very remote, and was cut off from Scotland by the intermediate styles of England and France. It should be noted that the pointed barrel vault, although very general, was not universally employed in Scotland during the third period. One or two notable examples of well constructed groined vaults are to be found, such as the vaulting of Trinity College Church and that of “Blackader’s Aisle” in Glasgow Cathedral. But these are exceptions to the general rule.