provided for during the casting of the slab. A thin facing of marble or
other ornamental solid or even plastic material may be applied to the
face of the slabs during the process of casting, thus enabling the work
to be finished as it is carried up, or a key may be formed on the face
of the slab to enable the structure to be plastered afterward.

[Illustration: FIG. 19. FIG 20.]

In Fig. 20, the structure from the bottom of the trenches is shown with
the sides of the trenches removed. It will be seen that the footings are
constructed in the most economical manner by not being stepped. As no
damp-course is required in concrete work, when the aggregate is of a
non-porous material, one is not shown. Upon the top of the footings is
generally laid a horizontal slab, called the wall-base slab, the special
feature of which is that it enables the thickness of the wall to be
gauged accurately, and also provides a fixing for the first course of
slabs. Figs. 4 and 5 show such slabs for internal and external angles,
and Fig. 6 shows one for straight work. The use of a wall-base slab is
not essential, although it is the more accurate method of building, for
in cases where it is desirable to economize labor, or from other causes,
the slabs forming the first course may be made with a thicker base, and
may be fixed by a deposition of concrete, which is allowed to set behind
them. The second course of slabs is laid upon the first course with
breaking joints of half-slab bond, each course being keyed to the other
by means of a quick-setting cementing material poured into the key-holes
provided in the edges of the slab for that purpose, a bituminous cement
being preferred. The key-holes are made in several ways, those shown in
the illustrations being of a dovetail shape; circular, square, or indeed
holes of any other shape formed in the edges of the slab and in an
oblique direction are also employed. Special slabs for cants, or