which this and all the Roman Catacombs were hewn seems as
if created specially for the purpose. Recent geological observations
have traced in the Campagna volcanic matter produced at different
periods, when the entire area of Rome and its vicinity was the seat of
active plutonic agency. This material is of varying degrees of
hardness. The lowest and oldest is so firm and compact that it still
furnishes, as it used to do, materials for building; the foundations
of the city, the wall of Romulus, and the massive blocks on which the
Capitol rests, being formed of this substance. Over this a later
stratum was deposited called _tufa granolare_, consisting of a similar
mechanical conglomerate of scoriae, ashes, and other volcanic products,
but more porous and friable in texture. It is in this last formation,
which is so soft that it can be easily hollowed out, and yet so solid
that it does not crumble, that the Catacombs are invariably found.
There is something that appeals strongly to the imagination in the
fact that the early Christians should have formed the homes of their
dead and the haunts of their faith in the deposit of the terrible
volcano and the stormy sea! The outbursts of the Alban volcanoes were
correlated in God's scheme of providence with the outbursts of human
fury long ages afterwards; and the one was prepared as a means of
defence from the other, by Him who maketh His ministers a flaming
fire.

The Catacombs were specially excavated for Christian burial,--tombs
beneath the tombs of the Appian Way. Unlike the pagans, who burned the
bodies of their dead, and deposited, as we have seen, the ashes in
cinerary urns which took up but little space, the Christians buried
the bodies of their departed friends in rock-hewn sepulchres. They
must have derived this custom from the Jewish mode of interment; and
they would wish to follow in this the example of their Lord, who was
laid in an excavated tomb. Besides, it was abhorrent to their feelings
to burn their dead. Their religion had taught them to value the body,
which is an integral part of human nature, and has its own share in
the redemption of man. Their mode of sepulture therefore required
larger space; and as the Christians grew and multiplied, and more
burials took place, they extended the subterranean passages and
galleries in every direction. It is computed that upwards of six
millions of the bodies of the early Christians were deposited in the
Catacombs. Th