lpidis," the sleeping place of Elpis;
"cubiculum Aureliae," the sleeping chamber of Aurelia. The term
catacomb was applied to these subterranean cemeteries at a much later
period. The practice of subterranean burial among the early Christians
was evidently derived from the Jewish custom of burying the dead in
excavated sepulchres, and thus may have been adopted by the early
Jewish converts. The Roman Jews had a very early catacomb of their
own, in the Monte Verde, contiguous to their place of abode, in the
Trasteverine quarter of Rome. This subterranean mode of sepulture is
undoubtedly of Egyptian origin. It is generally supposed that the
early Christians used for their burial places the excavations made by
the Romans for procuring stone and cement for building purposes. This
is an erroneous view. Recent geological observations on the soil of
the Agro Romano have shown that the surface of the Campagna consists
of volcanic rocks of different natures and ages. The earliest of the
series, the tufa lithoide, was constantly employed from the earliest
ages in the buildings of the city, as attested by the massive blocks
of the Cloaca Maxima, the tabularium of the Capitol, and the walls of
Romulus; the second, or tufa granolare, which though it has just
consistency enough to retain the form given to it by the excavator,
can not be hewn or extracted in blocks; and the pozzolana, which has
been extensively used in all ages for mortar or Roman cement. The tufa
lithoide and the pozzolana were thus alone used for building purposes
by the Romans, and the catacombs are never found excavated in these.
The catacombs were hewn only in the tufa granolare, and were
consequently excavated expressly for burials by the early Christians.
The Christian architects carefully avoided the massive strata of the
tufa lithoide, and we believe it is ascertained that all the known
catacombs are driven exclusively along the courses of the tufa
granolare. With equal care these subterranean engineers avoided the
layers of pozzolana, which would have rendered their work insecure,
and in which no permanent rock tomb could have been constructed. Thus
we arrive at the curious fact, that in making the catacombs the
excavators carefully avoided the strata of hard stone and the strata
of soft stone, used respectively for building and for mortar, and
selected that course of medium hardness which was best adapted to
their peculiar purpose. The early Christian tomb i