fully imitated by artists of the Cosmati family
and their school; and the mosaic pavements of this kind in the
medieval churches of Rome are no older than this period. But we have
reason to believe that the _Opus Alexandrinum_ in two of the chapels
of Santa Maria degli Angeli was taken from the Baths of Diocletian;
while the splendid pavement of the whole church, naves, transept, and
choir of Santa Croce in Jerusalemme, formed originally part of the
decorations of the Sessorian Palace of Sextus Varius, the father of
Heliogabalus, after whom the church is sometimes called the Sessorian
Basilica. The flooring of the whole upper church of San Clemente was
transferred from the older subterranean church, which derived its
pavement from some of the ruins of the Palatine or the Forum; and the
serpentine fragments, which enter very largely into the composition of
the curious old mosaic floor of Ara Coeli must have had a similar
origin as far back as the time of its founder, Gregory the Great. The
_Lapis Lacedaemonius_ must have been very abundant in Rome during the
time of Alexander Severus--judging from the quantities that are made
up into mosaics in the churches, and the heaps of broken fragments
that are found on the Palatine and at the Marmorata. The circular
space around the obelisk in the Piazza of St. Peter's to a
considerable extent is paved with it; and specimens of it frequently
occur among the ordinary road-metal in the city and neighbourhood.

Sicilian jaspers, so called, though really marbles, and purely
calcareous, because of their resemblance in colour and form of the
blotches to jasper, were wrought in great variety in the quarries in
the neighbourhood of the celebrated Taormina, and were transported in
the form of columns to Rome. Siliceous jaspers, obtained from the
crystalline rocks of Asia Minor, Egypt, and Northern Italy, were also
used for columns; and their brilliant red, green, and yellow hues,
highly polished, contrasted beautifully with the white marbles of the
interiors of the palaces. An even more sumptuous material called
_Murrha_ was employed, which has been identified with fluor-spar, a
translucent crystalline stone marked with blue, red, and purple,
similar to the beautiful substance found near Matlock in Derbyshire.
Of this fluor-spar were formed the celebrated murrhine cups which
were in use in Rome in the days of Pliny among the richest people, and
for which fabulous prices were paid. Several