re: http://www.freerepublic.com/focus/chat/3600146/posts?page=16#16
https://theodora.com/encyclopedia/c/cassiterides.html
CASSITERIDES (from the Gr. Kaaaimpos, tin, i.e. “ Tinislands”), in ancient geography the name of islands regarded as being situated somewhere near the west coasts of Europe. Herodotus (430 B.C.) had dimly heard of them. Later writers, Posidonius, Diodorus, Strabo and others, call them smallish islands off (Strabo says, some way off) the north-west coast of Spain, which contained tin mines, or, as Strabo says, tin and lead mines - though a passage in Diodorus derives the name rather from their nearness to the tin districts of north-west Spain. While geographical knowledge of the west was still scanty and the secrets of the tin-trade were still successfully guarded by the seamen of Gades and others who dealt in the metal, the Greeks knew only that tin came to them by sea from the far west, and the idea of tin-producing islands easily arose. Later, when the west was better explored, it was found that tin actually came from two regions, north-west Spain and Cornwall. Neither of these could be called “small islands” or described as off the north-west coast of Spain, and so the Cassiterides were not identified with either by the Greek and Roman geographers. Instead, they became a third, ill-understood source of tin, conceived of as distinct from Spain or Britain. Modern writers have perpetuated the error that the Cassiterides were definite spots, and have made many attempts to identify them. Small islands off the coast of north-west Spain, the headlands of that same coast, the Scillies, Cornwall, the British Isles as a whole, have all in turn been suggested. But none suits the conditions. Neither the Spanish islands nor the Scillies contain tin, at least in serious quantities. Neither Britain nor Spain can be called “small islands off the north-west of Spain.” It seems most probable, therefore, that the name Cassiterides represents the first vague knowledge of the Greeks that tin was found overseas somewhere in or off western Europe.
I blame the Phoenicians. They controlled the tin trade from Western Europe and they would seed false rumours of what lay beyond the pillars of Hercules to deter anyone else from exploring past that point and endangering their monopoly. “Tin islands” sounds like something they came up with to keep people looking in the wrong place.