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
Commerce - Manufactures
With this regular maritime intercourse between the British and Gallic coasts, the very close political connection between the inhabitants on both sides of the Channel is as easily explained as the flourishing of transmarine commerce and of fisheries. It was the Celts of Brittany in particular, that brought the tin of the mines of Cornwall from England and carried it by the river and land routes of Gaul to Narbo and Massilia. The statement, that in Caesar's time certain tribes at the mouth of the Rhine subsisted on fish and birds' eggs, may probably refer to the circumstance that marine fishing and the collection of the eggs of sea-birds were prosecuted there on an extensive scale.
When we put together and endeavour to fill up the isolated and scanty statements which have reached us regarding the Celtic commerce and intercourse, we come to see why the tolls of the river and maritime ports play a great part in the budgets of certain cantons, such as those of the Haedui and the Veneti, and why the chief god of the nation was regarded by them as the protector of the roads and of commerce, and at the same time as the inventor of manufactures. Accordingly the Celtic industry cannot have been wholly undeveloped; indeed the singular dexterity of the Celts, and their peculiar skill in imitating any model and executing any instructions, are noticed by Caesar.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=22