It was in the tenth year of the above-mentioned reign that Origen removed from Alexandria to Cæsarea, leaving the charge of the catechetical school in that city to Heraclas. Not long afterward Demetrius, bishop of the church of Alexandria, died, having held the office for forty-three full years, and Heraclas succeeded him. At this time Firmilianus, bishop of Cæsarea in Cappadocia, was conspicuous.
Historia Ecclesiastica
Book VI, Chapter 26. Heraclas becomes Bishop of Alexandria
1 section
Eusebius of Caesarea
Trans. Arthur Cushman McGiffert, A Select Library of Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume I, 1890. Public domain.