The Composition of Cassius Dio's 'Roman History'

I SPENT TEN YEARS in collecting all the achievements of the Romans from the beginning to the death of Severus, and another twelve in writing my history: subsequent events will be recorded to wherever it may be possible." So Cassius Dio, in the summary by Xiphilinus, describes the composition of his Roman History (72.23.5). To what period of twentytwo years does he refer? Until very recently, modern scholars who wrote about Dio adopted chronologies which exhibited an unusually high degree of unanimity. Eduard Schwartz, in what long remained the authoritative study of the historian (published in 1899), dated the beginning of Dio's enterprise to 194, the year after the accession of Septimius Severus, the completion of Book LXXVII, which ended with Severus' death, to 216. Schwartz, however, also believed that Dio subsequently made substantial additions, including the whole of Book LII, in the reign of Severus Alexander.1 Emilio Gabba in 1955 and Fergus Millar in 1964 adopted a chronology very close to that of Schwartz, but without the concession that Dio revised his History systematically: Gabba and Millar identified the twenty-two years as 196-218 and 197-219 respectively, and allowed no later revision to Books I-LXXVII beyond the addition of a very few brief sentences or clauses.2 Even G. Vrind, who wished to bring the decade during which Dio collected material down to the death of Severus to an end after 4 February 211, made the minimum modification necessary to Schwartz's chronology