The Catacombs are excavated in the volcanic rock which abounds in the neighborhood of Rome. It is a granulated, grayish breccia, or tufa, as it is called, of a coarse, loose texture, easily cut with a knife, and bearing still the marks of the mattocks with which it was dug. In the firmer volcanic rock of Naples the excavations are larger and loftier than those of Rome; but the latter, although they have less of apparent majesty, have more of funereal mystery. The Catacombs consist essentially of two parts—corridors and chambers, or cubicula. The corridors are long, narrow and intricate passages,
forming a complete underground net-work. They are for the most part straight, and intersect each other at approximate right angles. The accompanying map of part of the Catacomb of Callixtus will indicate the general plan of these subterranean galleries.
Fig. 3.—Part of Catacomb of Callixtus.
Fig. 4.—Gallery with Tombs.
The main corridors vary from three to five feet in width, but the lateral passages are much narrower, often
affording room for but one person to pass. They will average about eight feet in height, though in some places as low as five or six, and in others, under peculiar circumstances, reaching to twelve or fifteen feet. The ceiling is generally vaulted, though sometimes flat; and the floor, though for the most part level, has occasionally a slight incline, or even a few steps, caused by the junction of areas of different levels, as hereafter explained. The walls are generally of the naked tufa, though sometimes plastered; and where they have given way are occasionally strengthened with masonry. At the corners of these passages there are frequently niches, in