and here
only, _narrowed their bases_, and we have seven of them above, to six
below.

Sec. XXVII. We come now to the most interesting portion of the whole east
end, the archivolt at the end of the northern aisle.

It was above stated, that the band of triangles was broken by two higher
arches at the ends of the aisles. That, however, on the northern side of
the apse does not entirely interrupt, but lifts it, and thus forms a
beautiful and curious archivolt, drawn opposite, in Plate V. The upper
band of triangles cannot rise together with the lower, as it would
otherwise break the cornice prepared to receive the second story; and
the curious zigzag with which its triangles die away against the sides
of the arch, exactly as waves break upon the sand, is one of the most
curious features in the structure.

It will be also seen that there is a new feature in the treatment of the
band itself when it turns the arch. Instead of leaving the bricks
projecting between the sculptured or colored stones, reversed triangles
of marble are used, inlaid to an equal depth with the others in the
brickwork, but projecting beyond them so as to produce a sharp dark line
of zigzag at their junctions. Three of these supplementary stones have
unhappily fallen out, so that it is now impossible to determine the full
harmony of color in which they were originally arranged. The central
one, corresponding to the keystone in a common arch, is, however, most
fortunately left, with two lateral ones on the right hand, and one on
the left.

Sec. XXVIII. The keystone, if it may be so called, is of white marble,
the lateral voussoirs of purple; and these are the only colored stones
in the whole building which are sculptured; but they are sculptured in a
way which, more satisfactorily proves that the principle above stated
was understood by the builders, than if they had been left blank. The
object, observe, was to make the archivolt as rich as possible; eight of
the white sculptured marbles were used upon it in juxtaposition. Had the
purple marbles been left altogether plain, they would have been out of
harmony with the elaboration of the rest. It became necessary to touch
them with sculpture as a mere sign of carefulness and finish, but at the
same time destroying their colored surface as little as possible. _The
ornament is merely outlined upon them with a fine incision_, as if it
had been etched out on their surface preparatory to being carved. In