[5] Ptolemy and Dionysios Periegetes mentioned them—the former as ten small islands in northwest Iberia far off the coast and arranged symbolically as a ring, and the latter in connection with the mythical Hesperides.

[5] Gavin de Beer has suggested[13] that Roger Dion had solved the puzzle[14] by bringing to bear a chance remark in Avienius' late poem Ora maritima, which is based on early sources: the tin isles were in an arm of the sea within sight of wide plains and rich mines of tin and lead, and opposite two islands – a further one, Hibernia, and a nearer one, Britannia.

"Before the estuary of the Loire became silted up in late Roman times, the Bay of Biscay led into a wide gulf, now represented by the lower reaches of the river Brivet[15] and the marshes of the Brière, between Paimboeuf and St. Nazaire, in which were a number of islands.

De Beer confirms the location from Strabo: the Cassiterides are ten islands in the sea, north of the land of the Artabrians in the northwest corner of Hispania.

E. Thomas from the French BRGM showed in a 2004 report that tin mines were probably operated by the Romans at La Hye, near Ploërmel.