Of the Sedition excited at Alexandria, and how George was slain
It is now proper to mention what took place in the churches under the same [emperor]. A great disturbance occurred at Alexandria in consequence of the following circumstance. There was a place in that city which had long been abandoned to neglect and filth, wherein the pagans had formerly celebrated their mysteries, and sacrificed human beings to Mithra. This being empty and otherwise useless, Constantius had granted to the church of the Alexandrians; and George wishing to erect a church on the site of it, gave directions that the place should be cleansed.
In the process of clearing it, an adytum of vast depth was discovered which unveiled the nature of their heathenish rites: for there were found there the skulls of many persons of all ages, who were said to have been immolated for the purpose of divination by the inspection of entrails, when the pagans performed these and such like magic arts whereby they enchanted the souls of men. The Christians on discovering these abominations in the adytum of the Mithreum, went forth eagerly to expose them to the view and execration of all; and therefore carried the skulls throughout the city, in a kind of triumphal procession, for the inspection of the people.
When the pagans of Alexandria beheld this, unable to bear the insulting character of the act, they became so exasperated, that they assailed the Christians with whatever weapon chanced to come to hand, in their fury destroying numbers of them in a variety of ways: some they killed with the sword, others with clubs and stones; some they strangled with ropes, others they crucified, purposely inflicting this last kind of death in contempt of the cross of Christ: most of them they wounded; and as it generally happens in such a case, neither friends nor relatives were spared, but friends, brothers, parents, and children imbrued their hands in each other's blood.
Wherefore the Christians ceased from cleansing the Mithreum: the pagans meanwhile having dragged George out of the church, fastened him to a camel, and when they had torn him to pieces, they burnt him together with the camel.
Source: Ecclesiastical History (New Advent)