These corridors or passages of the sand-pits from which the Pozzolana sand had been excavated are large enough to admit a horse and cart; these were frequently the entrances to the catacombs, the corridors of which are usually by the side of or under those of the arenariæ, or sand-pits, and are only just large enough for a man, or two men with a body, to pass along; the height varies from five to seven or eight feet, or more, according to the thickness of the bed of tufa. In the catacomb of S. Hermes, part of the wide sand-pit road has been reduced to one-third of its width, by building up brick walls on each side with loculi in them.
There is in general, at present, no communication between one catacomb and another; each occupies a separate hill or rising ground in the Campagna, and is separated from the others by the intervening valleys. When the first tier of tombs extended to the edges of the hill, a second was made under it, and then sometimes a third, or more. The manner in which the rock is excavated in a number of corridors twisting in all directions, in order to make room for the largest possible number of bodies, is thus accounted for. The plan of the catacomb of S. Priscilla is a good illustration of this. It would have been hardly safe to have excavated the rock to any greater extent. The lowest corridors are frequently below the level of the valleys, and there may have been originally passages from one to the other, so that one entrance to S. Calixtus may have been through S. Sebastian's. The peculiarly dry and drying nature of the sandstone, or tufa rock, in which these tombs are excavated, made them admirably calculated for the purpose. These catacombs were the public cemeteries of Christian Rome for several centuries, and it would have been well for the health of the city if they could always have continued so. Unfortunately after the siege of Rome by the Goths, in the time of Justinian, when some of the catacombs were rifled of their contents, the use of these excellent burying places was discontinued.
That the arenaria were considered as burying places in the time of Nero is evident from his exclamations of horror at the idea of being taken there alive for the purpose of concealment. The sand-pits are also mentioned by Cicero in his Oration for Cluentius, where he says that the young Asinius, a citizen of noble family, was inveigled into one of them and murdered.
STONE COFFIN.[ToList]
This shows they were in use before the Christian era, and there is every reason to believe that they have been in use ever since lime-mortar came into use, which is believed to have been many centuries before that period. The celebrated Pozzolana sand makes the best mortar in the world, from its gritty nature. This valuable sand is found to any extent nearly all over the Campagna of Rome, in horizontal beds or layers between the beds of tufa; some of the tufa itself, which is sandstone, may be scraped into this sand, but it is easier to take it as ready provided by nature. People once accustomed to the use of this sand can not do without it, and hundreds of carts filled with it may be seen daily traversing the Campagna, conveying it either to Rome, or to Ostia, or to Porto, for exportation. The horizontal layers or beds of this sand are not usually more than six feet thick, although they extend at a certain level over the whole surface of the country. It is therefore excavated in horizontal corridors, with various branches, extending for many miles, undermining the whole surface of the soil, but not in large or deep pits, so that the name of sand-pit is rather deceitful to American people, who commonly imagine it to be always a large and deep pit to which these roads lead only; this is not always the case, the roads themselves being excavated in the layer of sand, and frequently themselves the sand-pits. Sometimes there are different layers of sand at different levels, and in some cases there may be two sand-pit roads one over the other, with the bed of hard tufa between them.
We are told in the Acta Sanctorum that one of the punishments inflicted on the Christians by the Emperor Maximinus in the sixth persecution, A.D. 35, was digging sand and stone. The martyrs, Ciriacus and Sisinnus are especially mentioned as ordered to be strictly guarded, and compelled to dig sand and to carry it on their own shoulders.
Some of the catacombs were evidently made under tombs by the side of the road, and in that of S. Calixtus there are remains of the tomb on the surface of the ground. The burial-chapels of the fourth century commonly found over a catacomb probably replace earlier tombs. The church of S. Urban is now considered to have been a family tomb of the first century, made into a church long afterwards.