obscure history and character of early Christian Art.]

The Roman catacombs consist for the most part of a subterranean
labyrinth of passages, cut through the soft volcanic rock of the
Campagna, so narrow as rarely to admit of two persons walking abreast
easily, but here and there on either side opening into chambers of
varying size and form. The walls of the passages, through their whole
extent, are lined with narrow excavations, one above another, large
enough to admit of a body being placed in each; and where they remain
in their original condition, these excavations are closed in front by
tiles, or by a slab of marble cemented to the rock, and in most cases
bearing an inscription. Nor is the labyrinth composed of passages upon a
single level only; frequently there are several stories, connected with
each other by sloping ways.

There is no single circumstance, in relation to the catacombs, of more
striking and at first sight perplexing character than their vast extent.
About twenty different catacombs are now known and are more or less
open,--and a year is now hardly likely to pass without the discovery
of a new one; for the original number of underground cemeteries, as
ascertained from the early authorities, was nearly, if not quite, three
times this number. It is but a very few years since the entrance to the
famous catacomb of St. Callixtus, one of the most interesting of all,
was found by the Cavaliere de Rossi; and it was only in the spring
of 1855 that the buried church and catacomb of St. Alexander on the
Nomentan Way were brought to light. Earthquakes, floods, and neglect
have obliterated the openings of many of these ancient cemeteries,--and
the hollow soil of the Campagna is full "of hidden graves, which men
walk over without knowing where they are."