In drawing sheet no. xxix. of my archæological map of Rome, which represents the region of the Sacra Via, I have had as much to do with Christian edifices as with pagan ruins.[92] VI. MEMORIALS OF HISTORICAL EVENTS. The first commemorative chapel erected in Rome is perhaps contemporary with the Arch of Constantine, and refers to the same event, the victory gained by the first Christian emperor over Maxentius in the plain of the Tiber, near Torre di Quinto. [Illustration: Statue of Constantine the Great.] The existence of this chapel, called the _Oratorium Sanctæ Crucis_ ("the oratory of the holy cross"), is frequently alluded to in early church documents. The name must have originated from a monumental cross erected on the battlefield, in memory of Constantine's vision of the "sign of Christ" (the monogram [Symbol: Christ]). In the procession which took place on S. Mark's day, from the church of S. Lorenzo in Lucina to S. Peter's, through the Via Flaminia and across the Ponte Milvio, the first halt was made at S. Valentine's,[93] the second at the chapel of the Holy Cross. The "Liber Pontificalis," in the Life of Leo III. (795-816), speaks of this strange ceremony. It was called the "great litany," and occurred on the twenty-third of April, the day on which the Romans used to celebrate the Robigalia. The Christian litany and the pagan ceremony had the same purpose, that of securing the blessing of Heaven upon the fields, and averting from them the pernicious effects of late spring frosts. The rites were nearly the same, the principal one being a procession which left Rome by the Porta Flaminia, and passed across the Ponte Milvio to a suburban sanctuary. The end of the pagan pilgrimage was a temple of the god Robigus or the goddess Robigo, situated at the fifth milestone of the Via Claudia; that of the Christian the monumental cross near the same road, and ultimately the basilica of S. Peter's. In course of time the oratory and cross lost their genuine meaning; they were thought to mark the spot on which the miraculous vision had appeared to Constantine on the eve of battle. This was not the case, however, because Eusebius, to whom the emperor himself described the event, says that the luminous sign appeared to him before the commencement of military operations, which means before he crossed the Alps and took possession of Susa, Turin, and Vercelli. But, if the heavenly apparition of the "sign of Christ" on Monte Mario is historically without foundation, the existence of the oratory is not. Towards the end of the twelfth century it was in a ruinous state, and converted probably into a stable or a hay-loft. The last archæologist who mentions it is Seroux d'Agincourt. He describes the ruins "on the slopes of the hill of the Villa Madama," and gives a sketch of the paintings which appeared here and there on the broken walls. Armellini and myself have explored the beautiful woods of the Villa Madama in