s, the corridors of
which are usually by the side of or under those of the _arenariae_, or
sand-pits, and are only just large enough for a man, or two men with
a body, to pass along; the height varies from five to seven or eight
feet, or more, according to the thickness of the bed of tufa. In the
catacomb of S. Hermes, part of the wide sand-pit road has been reduced
to one-third of its width, by building up brick walls on each side
with _loculi_ in them.

There is in general, at present, no communication between one catacomb
and another; each occupies a separate hill or rising ground in the
Campagna, and is separated from the others by the intervening valleys.
When the first tier of tombs extended to the edges of the hill, a
second was made under it, and then sometimes a third, or more. The
manner in which the rock is excavated in a number of corridors
twisting in all directions, in order to make room for the largest
possible number of bodies, is thus accounted for. The plan of the
catacomb of S. Priscilla is a good illustration of this. It would have
been hardly safe to have excavated the rock to any greater extent. The
lowest corridors are frequently below the level of the valleys, and
there may have been originally passages from one to the other, so that
one entrance to S. Calixtus may have been through S. Sebastian's. The
peculiarly dry and drying nature of the sandstone, or tufa rock, in
which these tombs are excavated, made them admirably calculated for
the purpose. These catacombs were the public cemeteries of Christian
Rome for several centuries, and it would have been well for the health
of the city if they could always have continued so. Unfortunately
after the siege of Rome by the Goths, in the time of Justinian, when
some of the catacombs were rifled of their contents, the use of these
excellent burying places was discontinued.

That the _arenaria_ were considered as burying places in the time of
Nero is evident from his exclamations of horror at the idea of being
taken there alive for the purpose of concealment. The sand-pits are
also mentioned by Cicero in his Oration for Cluentius, where he says
that the young Asinius, a citizen of noble family, was inveigled into
one of them and murdered.

[Illustration: STONE COFFIN.]

This shows they were in use before the Christian era, and there is
every reason to believe that they have been in use ever since
lime-mortar came into use, which is believed to h