About the time that Alexander was making his way eastward through Persia, Pytheas was leaving the Greek colony of Marseilles for the west and north. The Phoenicians, with their headquarters at Carthage, had complete command of the mineral trade of Spain--the Mexico of the ancient world. They knew where to find the gold and silver from the rivers--indeed, they said that the coast, from the Tagus to the Pyrenees, was "stuffed with mines of gold and silver and tin." The Greeks were now determined to see for themselves--the men of Carthage should no longer have it all their own way. Where were these tin islands, kept so secret by the master-mariners of the ancient world? A committee of merchants met at Marseilles and engaged the services of Pytheas, a great mathematician, and one who made a study of the effect of the moon on the tides. All sorts of vague rumours had reached the ears of Pytheas about the northern regions he was about to visit. He would discover the homes of the tin and amber merchants, he would find the people who lived "at the back of the north wind," he would reach a land of perpetual sunshine, where swans sang like nightingales and life was one unending banquet. So Pytheas, the mathematician of Marseilles started off on his northern trip. Unfortunately, his diary and book called _The Circuit of the Earth_ have perished, and our story of geographical discovery is the poorer. But these facts have survived. The ships first touched at Cadiz, the "Tyre of the West," a famous port in those days, where Phoenician merchants lived, "careless and secure" and rich. This was the limit of Greek geographical knowledge; here were the Pillars of Hercules, beyond which all was dim and mysterious and interesting. Five days' sail, that is to say, some three hundred miles along the coast of Spain, brought Pytheas to Cape St. Vincent. He thought he was navigating the swift ocean river flowing round the world. He was, therefore, surprised to find as he rounded the Cape that the current had ceased, or, in his own words, the "ebb came to an end." Three days more and they were at the mouth of the Tagus. Near this part of the coast lay the Tin Islands, according to Greek ideas, though even to-day their exact locality is uncertain. Pytheas must have heard the old tradition that the Cassiterides were ten in number and lay near each other in the ocean, that they were inhabited by people who wore black cloaks and long tunics reaching to the feet, that they walked with long staves and subsisted by their cattle. They led a wandering life; they bartered hides, tin, and lead with the merchants in exchange for pottery, salt, and implements of bronze.