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; it preserved some of the graves from the rifling to which most were exposed during the period of the desertion of the catacombs.  Most of the graves which are now found with their tiled or marble front complete, and with the inscription of name or date upon them unbroken, are those which were thus secluded.

But there is still another curious fact bearing upon the Christian origin of the catacombs.  They are in general situated on somewhat elevated land, and always on land protected from the overflow of the river, and from the drainage of the hills.  The early traditions of the Church preserve the names of many Christians who gave land for the purpose,—­a portion of their vignas, or their villas.  The names of the women Priscilla, Cyriaca, and Lucina are honored with such remembrance, and are attached to three of the catacombs.  Sometimes a piece of land was thus occupied which was surrounded by property belonging to those who were not Christian.  This seems to have been the case, for instance, in regard to the cemetery of St. Callixtus; for (and this is one of the recent discoveries of the Cavaliere de Rossi) the paths of this cemetery, crossing and recrossing in three, four, and five stages, are all limited to a definite and confined area,—­and this area is not determined by the quality of the ground, but apparently by the limits of the field overhead.  There can be no other probable explanation of this but that Christians would not extend their burial-place under land that was not in their possession.  Many other facts, as we shall see in other connections, go to establish beyond the slightest doubt the Christian origin and occupation of the catacombs.