n ruin; very different, too, from the _columbaria_, or pigeon-holes,
in which the ashes of the less wealthy were packed away; and still more
different from the sad undistinguished ditch that received the bodies of
the poor:--

"Hoc miserae plebi stabat commune sepulcrum."

[Footnote D: The volcanic rocks are the _Tufa litoide_, very hard, and
used for paving and other such purposes; difficult to be quarried, and
unfit for graves on account of this difficulty. The _Tufi granulare_, a
soft, friable, coarse-grained rock, easily cut,--fitted for excavation.
It is in this that the catacombs are made. It is used for very few
purposes in Rome. One may now and then see some coarse filling-up of
walls done with it, or its square-cut blocks piled up as a fence. The
third is the _Pura pozzolana_,--which is the _Tufa granulare_ in a state
of compact sand, yielding to the print of the heel, dug like sand, and
used extensively in the unsurpassed mortar of the Roman buildings.]

It not unfrequently happens in the soil of the Campagna, that the vein
of harder rock in which the catacombs are quarried assumes the soft and
sandy character which belongs to it in a state of decomposition. The
ancient Romans dug this sand as the modern Romans do; and it seems
probable, from the fact that some of the catacombs open out into
_arenaria_, or sandpits, as in the case of the famous one of St. Agnes,
that the Christians, in time of persecution, when obliged to bury with
secresy, may have chosen a locality near some disused sandpit, or near a
sandpit belonging to one of their own number, for the easier concealment
of their work, and for the safer removal of the quarried tufa. In such
cases the tufa may have been broken down into the condition of sand for
removal. In later times, as the catacombs were extended, the tufa dug
out from one passage was carried into the old passages no longer used;
and thus, as the catacomb extended in one direction, it was closed up in
another, and the ancient graves were concealed. This is now one of the
great impediments in the way of modern exploration; and the same process
is being repeated at present; for the Church allows none of the earth or
stone to be removed that has been hallowed as the resting-place of the
martyrs, and thus, as one passage is now opened, another has to be
closed. The archaeologists may rebel, but the priests have their way.
The ancient filling up was, however, productive of one good result