large spaces of flat or dead wall, rising above the aisles, and forming
the upper part of the nave, now called the clerestory, which had a
gabled wooden roof.

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