Their origin was, in the last century, a subject of controversy. Following two or three vague and equivocal passages, some learned writers pronounced the catacombs to have been originally heathen excavations, made to extract sand for the building of the city. These sand-pits were called _arenaria_, and so occasionally are the Christian cemeteries. But a more scientific and minute examination, particularly made by the accurate F. Marchi, has completely confuted this theory. The entrance to the catacombs was often, as can yet be seen, from these sand-pits, which are themselves under ground, and no doubt were a convenient cover for the cemetery; but several circumstances prove that they were never used for Christian burial, nor converted into Christian cemeteries. The man who wishes to get the sand out of the ground will keep his excavation as near as may be to the surface; will have it of easiest possible access, for drawing out materials; and will make it as ample as is consistent with the safety of the roof, and the supply of what he is seeking. And all this we find in the _arenaria_ still abounding round Rome. But the catacombs are constructed on principles exactly contrary to all these. The catacomb dives at once, generally by a steep flight of steps, below the stratum of loose and friable sand,[87] into that where it is indurated to the hardness of a tender, but consistent rock; on the surface of which every stroke of the pickaxe is yet distinctly traceable. When you have reached this depth you are in the first story of the cemetery, for you descend again by stairs to the second and third below, all constructed on the same principle. A catacomb may be divided into three parts, its passages or streets, its chambers or squares, and its churches. The passages are long, narrow galleries, cut with tolerable regularity, so that the roof and floor are at right angles with the sides, often so narrow as scarcely to allow two persons to go abreast. They sometimes run quite straight to a great length; but they are crossed by others, and these again by others, so as to form a complete labyrinth, or net-work, of subterranean corridors. To be lost among them would easily be fatal. But these passages are not constructed, as the name would imply, merely to lead to something else. They are themselves the catacomb or cemetery. Their walls, as well as the sides of the staircases, are honeycombed with graves, that is, with rows of excavations, large and small, of sufficient length to admit a human body, from a child to a full-grown man, laid with its side to the gallery. Sometimes there are as many as fourteen, sometimes as few as three or four, of these rows, one above the other. They are evidently so made to measure, that it is probable the body was lying by the side of the grave, while this was being dug.