The ever-pressing necessity for graves compelled the fossors at length to attempt the construction of galleries on a lower level. Accordingly we find a stairway, H, H2, of thirty-four steps leading down from the gallery H. The rock, however, through which this stairway descends is no longer the firm _tufa granolare_ of the upper level, but a very friable stratum of _pozzolana_, which made it necessary to protect the walls with brick-work. Finding this stratum of great depth, they excavated a horizontal passage, and a still further narrow experimental cleft, as it were, in search of firmer rock, but soon abandoned the attempt, failing to find any suitable for sepulture. The few graves they made had to be built of brick-work; and in one of 173 these was found a little terra cotta sarcophagus, containing the body of an infant. This shows the utter unfitness of the _pozzolana_ beds in which the _arenaria_ are excavated for the construction of the Catacombs. We have seen that about A. D. 200 Callixtus became the guardian of this cemetery, which seems to have then become the burial-place of the bishops of Rome instead of the crypts of the Vatican as previously. According to the _Liber Pontificalis_, out of eighteen bishops from Zephyrinus to Sylvester, that is, from A. D. 197 to A. D. 314, no less than thirteen were buried in this cemetery. This Callixtus was originally a slave, afterwards elevated to the highest ecclesiastical dignities, including the episcopate itself--a proof of the superiority of the church to all social distinctions. According to Hippolytus, the undoubted author of the recently discovered _Philosophoumena_, he reached that dignity by dishonourable means, by fraud and guile. He was at one time banished by the emperor to the mines of Sardinia for embezzling moneys intrusted to his care, and on his return lapsed into heresy bordering on pantheism, or at least was charged with that offence. But although the character of Callixtus shows the nascent corruptions of the church of Rome even early in the third century, it should not prejudice us against the cemetery called by his name. He himself is interred elsewhere,[301] and the holy confessors and martyrs who slumbered here have consecrated the place forever with their hallowed dust.