The Ecclesiastical History of Socrates Scholasticus 74Of Gallus Cæsar.

Gallus, having accomplished these things, was unable to bear his success with moderation; but forthwith attempted innovations against the authority of him who had constituted him Caesar, himself aspiring to the sovereign power. His purpose was, however, soon detected by Constantius: for he had dared to put to death, on his own responsibility, Domitian, at that time Praetorian prefect of the East, and Magnus the quaestor, not having disclosed his designs to the emperor. Constantius, extremely incensed at this conduct, summoned Gallus to his presence, who being in great terror went very reluctantly; and when he arrived in the western parts, and had reached the island of Flanona, Constantius ordered him to be slain. But not long after he created Julian, the brother of Gallus, Caesar, and sent him against the barbarians in Gaul. It was in the seventh consulate of the emperor Constantius that Gallus, who was surnamed Constantius, was slain, when he himself was a third time consul: and Julian was created Caesar on the 6th of November in the following year, when Arbetion and Lollian were consuls; of him we shall make farther mention in the next book. When Constantius was thus relieved from the disquietudes which had occupied him, his attention was again directed to ecclesiastical contentions. Going therefore from Sirmium to the imperial city Rome, he again appointed a synod of bishops, summoning some of the eastern prelates to hasten into Italy, and arranging for those of the west to meet them there. While preparations were making in the east for this purpose, Julius bishop of Rome died, after having presided over the church in that place fifteen years, and was succeeded in the episcopal dignity by Liberius.