that it will not
keep its polish without extreme care; a circumstance which
distinguishes it from the Rhodian marble, whose tenacity in this
respect renders it eminently adapted for the more costly class of
decorative works.

The marbles we have been hitherto considering belong to the older
calcareous formations of Italy, Greece, Asia Minor, and Egypt, and go
down to the upper triassic and muschel-kalk limestones, and perhaps
even to those of an older period. But there is a class of ancient
marbles in Rome of much more recent geological origin--belonging
indeed to the Miocene epoch--which are called Lumachella, from the
Italian word signifying snail, on account of the presence in all the
species of fossil shells. They vary in colour from the palest straw to
the deepest purple. Some of them are exceedingly beautiful and
valuable, and they are nearly all more or less rare, being found
chiefly in small fragments of ancient pavements. Their substance is
formed of the shells of the common oyster in bluish gray and black
particles on a white ground, as in the Lumachella d' Egitto; of the
cardium or cockle, assuming a lighter or deeper shade of yellow, as in
the Lumachella d' Astracane; of the ammonite, as in the L. Corno d'
Ammone; of the Anomia ampulla in the L. occhio di Pavone, so called
from the circular form of the fossils whichever way the section is
made; of encrinites, belemnites, and starfish, showing white or red on
a violet ground, as in the L. pavonazza; and "of broken shells, hardly
discernible, together with very shining and saccharoid particles of
carbonate of lime," as in the _Marmor Schiston_ of the ancients--the
_brocatello antico_ of the Italians, so named from its various shades
of yellow and purple, resembling silk brocade. The most important
specimens of Lumachella marbles are the pair of very fine large
columns of L. rosea on the ground-floor of the Schiarra Palace, the
balustrade of the high altar of St. Andrea della Valle, two columns in
the garden of the Corsini Palace of L. d' Astracane, and a pair of
large pillars which support one of the arches of the Vatican Library,
formed of L. occhio di pavone. Specimens of brocatello may be found in
several churches and palaces, forming mouldings, sheathings, and
pedestals.

The most interesting of the Lumachella marbles is the _bianca antica_,
the Marmor Megarense of the ancients, composed of shells so small as
to be scarcely discernible, and so closely