These high dead walls were, in Roman work, built of stone; but in the wooden work of the North, they must necessarily have been made of horizontal boards or timbers attached to uprights on the top of the nave pillars, which were themselves also of wood. [Footnote: Appendix 9, "Wooden Churches of the North."] Now, these uprights were necessarily thicker than the rest of the timbers, and formed vertical square pilasters above the nave piers. As Christianity extended and civilization increased, these wooden structures were changed into stone; but they were literally petrified, retaining the form which had been made necessary by their being of wood. The upright pilaster above the nave pier remains in the stone edifice, and is the first form of the great distinctive feature of Northern architecture--the vaulting shaft. In that form the Lombards brought it into Italy, in the seventh century, and it remains to this day in St. Ambrogio of Milan, and St. Michele of Pavia. SECTION XXVIII. When the vaulting shaft was introduced in the clerestory walls, additional members were added for its support to the nave piers. Perhaps two or three pine trunks, used for a single pillar, gave the first idea of the grouped shaft. Be that as it may, the arrangement of the nave pier in the form of a cross accompanies the superimposition of the vaulting shaft; together with corresponding grouping of minor shafts in doorways and apertures of windows. Thus, the whole body of the Northern architecture, represented by that of the Lombards, may be described as rough but majestic work, round-arched, with grouped shafts, added vaulting shafts, and endless imagery of active life and fantastic superstitions. SECTION XXIX. The glacier stream of the Lombards, and the following one of the Normans, left their erratic blocks, wherever they had flowed; but without influencing, I think, the Southern nations beyond the sphere of their own presence. But the lava stream of the Arab, even after it ceased to flow, warmed the whole of the Northern air; and the history of Gothic architecture is the history of the refinement and spiritualization of Northern work under its influence. The noblest buildings of the world, the Pisan-Romanesque, Tuscan (Giottesque) Gothic, and Veronese Gothic, are those of the Lombard schools themselves, under its close and direct influence; the various Gothics of the North are the original forms of the architecture which the Lombards brought into Italy, changing under the less direct influence of the Arab.