Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Brittany was in Caesar's time a country poor in corn. In the north-east dense forests, attaching themselves to the heart of the Ardennes, stretched almost without interruption from the German Ocean to the Rhine; and on the plains of Flanders and Lorraine, now so fertile, the Menapian and Treverian herdsman then fed his half-wild swine in the impenetrable oak-forest. Just as in the valley of the Po the Romans made the production of wool and the culture of corn supersede the Celtic feeding of pigs on acorns, so the rearing of sheep and the agriculture in the plains of the Scheldt and the Maas are traceable to their influence.
In Britain even the threshing of corn was not yet usual; and in its more northern districts agriculture was not practised, and the rearing of cattle was the only known mode of turning the soil to account. The culture of the olive and vine, which yielded rich produce to the Massiliots, was not yet prosecuted beyond the Cevennes in the time of Caesar.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=18