The igneous formation of which
the greater part of the Roman Campagna is in its superior
portion composed contains three strata known under the common
name of tufa: the stony, granular, and sandy tufa, the last
being commonly known as pozzolana. The pozzolana
is the material required for building purposes, for
admixture
with mortar; and the sandpits are naturally excavated in the
stratum which supplies it.
The stony tufa (tufa litoide) is quarried as building
stone. The granular tufa is useless for either purpose
containing too much earth to be employed in making mortar, and
being far too soft to be used as stone for building. Yet it is
in this stratum, and in this alone, that the catacombs are
constructed; their engineers avoiding with equal care the
solid stone of the tufa litoide and the friable pozzolana,
and selecting the stratum of medium hardness, which enabled
them to form the vertical walls of their galleries, and to excavate
the loculi and cubicula without severe labor and also without
fear of their falling in.
The modifications required to
strengthen the crumbling walls to support the roof and to
facilitate the excavation of loculi involved so much labor
that as a rule after a few attempts, the idea of utilizing an
old quarry for burial purposes was abandoned.
The mode of formation of the
catacombs almost without exception had their origin in small burial areas, the property of private persons or
of families, gradually ramifying and receiving additions of one subterranean
story after another as each was required for interments.
The first step would be the acquisition of a plot of ground either by gift or purchase for the formation of a tomb. Christians were
not beyond the pale of the law, and their faith presented no hindrance to the property being secured to them in perpetuity. To adapt the ground for its purpose as a cemetery, a gallery was run all round the area in the tufa rock at a convenient depth below the surface, reached by staircases at the corners. In the upright walls of these galleries loculi
were cut as needed to receive the dead. When these first four galleries were full others were mined on the same
level at right angles to them, thus gradually converting the whole area into a
network of corridors. If a family vault was required, or a burial chapel for a martyr or person of distinction, a small square room was excavated by the side of the gallery and communicating with it. When the original area had been mined in this way as far as was consistent with stability, a second
story of galleries was begun at a lower level, reached by a new staircase. This was succeeded by a third, or a fourth, and sometimes even by a fifth. When adjacent burial areas belonged to members of the same Christian confraternity, or by gift or purchase fell into the same hands, communications were opened between the respective cemeteries, which thus spread laterally, and gradually acquired that enormous extent which,
even when their fabulous dimensions are reduced to their right
measure form an immense work.
This could only be executed by a large and powerful
Christian community unimpeded by legal enactments or police
regulations. But although in ordinary times there was no necessity for secrecy,
when the peace of the Church was broken by the fierce and often protracted persecutions of the
pagan emperors, it became essential to adopt precautions to conceal the entrance to the
cemeteries which became the temporary hiding places of the Christian fugitives, and to baffle the search of their pursuers. To these stormy periods we may safely assign the alterations which may be traced in the staircases, which are sometimes abruptly cut off, leaving a gap requiring a ladder, and the
formation of secret passages communicating with the
arenariae, and through them with the open country.
When the storms of persecution ceased and Christianity had become the imperial faith, the fruits of prosperity were not slow to appear. Cemetery interment became a regular trade
in the hands of the fossores, or grave-diggers, who appear to have established a kind of property in the
catacombs and whose greed of gain led to that destruction of the religious paintings with which the walls were
decorated for the quarrying of fresh loculi. Monumental epitaphs record the purchase of a grave from the
fossores, in many cases during the lifetime of the individual, not
infrequently stating the price. A very curious fresco found in the cemetery of
Calixtus, preserved by the engravings of the earlier investigators
represents a “fossor” with his lamp in his hand and his pick over his shoulder, and his tools lying about him. Above is the inscription,
Diogenes
Fossor in Pace depositus.