Three different kinds of stone compose the basis of the Roman Campagna; the _tufa litoide_, as hard and durable as granite, used extensively for building purposes; the _tufa granolare_, which is consistent enough to retain the form given to it by excavators, but it is useless as building material, and lastly the _Pozzuolana_, largely employed in the making of Roman cement. Neither the _arenaria_ or sand quarries, nor those for the building stone were ever employed for excavation to make catacombs, whereas the _granular tufa_ has been so largely excavated for this purpose that if the galleries were continued in one line, it has been reckoned that they would stretch the entire length of the Italian peninsula. They form a labyrinth of passages and cross-passages, and are moreover in several stages called _piani_. But they do not extend far from the Eternal City, not beyond the third milestone. The galleries have a breadth of from two to four feet, and their height is governed by the nature of the rock in which they are hewn. The walls on both sides are lined with graves dug out of the rock, in a horizontal position, one above the other, like bunks in a cabin. In each of these reposed one or more bodies. Here and there the sequence is broken by a cross-passage that leads to a small chamber, and in these chambers the sides, like those of the galleries, are perforated with graves. All these graves were originally closed by slabs of marble or tiles. This is about the only distinction between the graves of the rich and those of the poor, of the slave from his master. Those who desired to set some mark on the resting-place of a relative, to distinguish it from those around, either had the name engraved upon the slab, or rudely scratched with the sharp end of a trowel in the mortar by which the slab was secured, or else a bit of ornamented glass or a ring or coin was impressed in the mortar while it was still wet. The martyrs in many cases were accorded a more elaborate grave. They were laid in a sarcophagus in an _arcossolium_, and on the covering slab the Holy Mysteries were celebrated on the anniversary of their martyrdom. But sometimes a wealthy family had its own chamber, _cubiculum_, reserved for its members. The puticoli, of which mention has already been made as ash and refuse pits, were of a totally different description. They were funnel-shaped shafts sunk in the rocks, the narrow orifice being on the level of the ground. Into this were precipitated the carcases of slaves and of the poor. Indeed, they are still in use at Naples, when a cart with a lantern may be followed till it reaches the place of interment, where a hole gapes. The corpse that is enveloped in a shroud only, is shot down into the hole, without its winding sheet, that is reserved for further use.