ief in the old traditional view, his own researches by
degrees opened his eyes to the truth, now universally recognized, that
the catacombs were exclusively the work of the Christians, and were
constructed for the interment of the dead. It is true that a catacomb is
often connected with the earlier sand-quarry, and starts from it as a
commencement, but the two are excavated in different strata, suitable to
their respective purposes, and their plan and construction are so
completely unlike as to render any confusion between them impossible.

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_.[3] 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 labour and also
without fear of their falling in. The annexed illustration (fig. 16)
from Marchi's work, when compared with that of the catacomb of Sant'
Agnese already given, presents to the eye the contrast between the wide
winding irregular passages of the sand-pit, calculated for the admission
of a horse and cart, and the narrow rectilinear accurately-defined
galleries of the catacomb. The distinction between the two is also
plainly exhibited when for some local or private reasons an ancient
_arenaria_ has been transformed into a cemetery. The modifications
required to strengthen the crumbling walls to support the roof and to
facilitate the excavation of _loculi_, involved so much labour that, as
a rule, after a few attempts, the idea of utilizing an old quarry for
burial purposes was abandoned.

[Illustration: FIG. 16.--Arenaria beneath the Cemetery of Calixtus.]

Another equal