f the fourth, second, or sixth order, the reader will
only have to refer to the numerals at the top of Plate XIV.

Then the series below shows the principal forms found in each period,
belonging to each several order; except 1 _b_ to 1 _c_, and the two
lower series, numbered 7 to 16, which are types of Venetian doors.

Sec. XXV. We shall now be able, without any difficulty, to follow the
course of transition, beginning with the first order, 1 and 1 _a_, in
the second row. The horse-shoe arch, 1 _b_, is the door-head commonly
associated with it, and the other three in the same row occur in St.
Mark's exclusively; 1 _c_ being used in the nave, in order to give a
greater appearance of lightness to its great lateral arcades, which at
first the spectator supposes to be round-arched, but he is struck by a
peculiar grace and elasticity in the curves for which he is unable to
account, until he ascends into the galleries whence the true form of the
arch is discernible. The other two--1 _d_, from the door of the
southern transept, and 1 _c_, from that of the treasury,--sufficiently
represent a group of fantastic forms derived from the Arabs, and of
which the exquisite decoration is one of the most important features in
St. Mark's. Their form is indeed permitted merely to obtain more fantasy
in the curves of this decoration.[85] The reader can see in a moment,
that, as pieces of masonry, or bearing arches, they are infirm or
useless, and therefore never could be employed in any building in which
dignity of structure was the primal object. It is just because structure
is _not_ the primal object in St. Mark's, because it has no severe
weights to bear, and much loveliness of marble and sculpture to exhibit,
that they are therein allowable. They are of course, like the rest of
the building, built of brick and faced with marble, and their inner
masonry, which must be very ingenious, is therefore not discernible.
They have settled a little, as might have been expected, and the
consequence is, that there is in every one of them, except the upright
arch of the treasury, a small fissure across the marble of the flanks.

[Illustration: Fig. XXVI.]

Sec. XXVI. Though, however, the Venetian builders adopted these Arabian
forms of arch where grace of ornamentation was their only purpose, they
saw that such arrangements were unfit for ordinary work; and there is no
instance, I believe, in Venice, of their having used any of them for a
dwelli