The work of Ammianus extended from the accession of Nerva, A. D. 96, the point at which the histories of Tacitus and the biographies of Suetonius terminated, to the death of Valens, A. D. 378, comprising a period of 282 years. It was divided into thirty-one books, of which the first thirteen are lost. The remaining eighteen embrace the acts of Constantius from A. D. 353, the seventeenth year of his reign, together with the whole career of Gallus, Julianus, Jovianus, Valentinianus, and Valens. The portion preserved includes the transactions of twenty-five years only, which proves that the earlier books must have presented a very condensed abridgment of the events contained in the long space over which they stretched; and hence we may feel satisfied, that what has been saved is much more valuable than what has perished.

Gibbon (cap. xxvi.) pays a well-deserved tribute to the accuracy, fidelity, and impartiality of Ammianus. We are indebted to him for a knowledge of many important facts not elsewhere recorded, and for much valuable insight into the modes of thought and the general tone of public feeling prevalent in his day. His history must not, however, be regarded as a complete chronicle of that era; those proceedings only are brought forward prominently in which he himself was engaged, and nearly all the statements admitted appear to be founded upon his own observations, or upon the information derived from trustworthy eye-witnesses. A considerable number of dissertations and digressions are introduced, many of them highly interesting and valuable. Such are his notices of the institutions and manners of the Saracens (14.4), of the Scythians and Sarmatians (17.12), of the Huns and Alani (31.2), of the Egyptians and their country (22.6, 14-16), and his geographical discussions upon Gaul (15.9), the Pontus (22.8), and Thrace (27.4), although the accuracy of many of his details has been called in question by D'Anville. Less legitimate and less judicious are his geological speculations upon earthquakes (17.7), his astronomical inquiries into eclipses (20.3), comets (25.10), and the regulation of the calendar (26.1), his medical researches into the origin of epidemics (19.4), his zoological theory on the destruction of lions by mosquitoes (18.7), and his horticultural essay on the impregnation of palms (24.3). But in addition to industry in research and honesty of purpose, he was gifted with a large measure of strong common sense which enabled him in many points to rise superior to the prejudice of his day, and with a clear-sighted independence of spirit which prevented him from being dazzled or overawed by the brilliancy and the terrors which enveloped the imperial throne. The wretched vanity, weakness, and debauchery of Constantius, rendering him an easy prey to the designs of the profligate minions by whom he was surrounded, the female intrigues which ruled the court of Gallus, and the conflicting elements of vice and virtue which were so strongly combined in the character of Valentinian, are all sketched with boldness, vigour, and truth. But although sufficiently acute in detecting and exposing the follies of others, and especially in ridiculing the absurdities of popular superstition, Ammianus did not entirely escape the contagion. The general and deep-seated belief in magic spells, omens, prodigies, and oracles, which appears to have gained additional strength upon the first introduction of Christianity, evidently exercised no small influence over his mind. The old legends and doctrines of the Pagan creed and the subtle mysticism which philosophers pretended to discover lurking below, when mixed up with the pure and simple but startling tenets of the new faith, formed a confused mass which few intellects, except those of the very highest class, could reduce to order and harmony.

A keen controversy has been maintained with regard to the religious creed of our author. (See Bayle.) There is nothing in his writings which can entitle us to decide the question positively. In several passages he speaks with marked respect of Christianity and its professors (xxi. sub fin., 22.11, 27.3; compare 22.12, 25.4); but even his strongest expressions, which are all attributed by Gibbon " to the incomparable pliancy of a polytheist," afford no conclusive evidence that he was himself a disciple of the cross. On the other hand he does not scruple to stigmatize with the utmost severity the savage fury of the contending sects (22.5), nor fail to reprobate the bloody violence of Damasus and Ursinus in the contest for the see of Rome (27.3) : the absence of all censure on the apostacy of Julian, and the terms which he employs with regard to Nemesis (14.11, 22.3), the Genius (21.14), Mercurius (16.5, 25.4), and other deities, are by many considered as decisive proofs that he was a pagan. Indeed, as Heyne justly remarks, many of the writers of this epoch seem purposely to avoid committing themselves. Being probably devoid of strong religious principles, they felt unwilling to hazard any declaration which might one day expose them to persecution and prevent them from adopting the various forms which the faith of the court might from time to time assume.