Up to this time their line of retreat had followed the "royal road" of merchants and caravans. Their only chance of safety lay in striking north into the mountains inhabited by this warlike tribe who had held out amid their wild and rugged country against the Persians themselves. They now opposed the Greeks with all their might, and it took seven days of continuous fighting to reach the valley which lay between them and the high tableland of Armenia. They crossed the Tigris near its source, and a little farther on they also crossed the Euphrates not far from its source, so they were informed by the Armenians. They now found themselves some five or six thousand feet above sea-level and in the midst of a bitter Armenian winter. Snow fell heavily, covering all tracks, and day after day a cold north-east wind, "whose bitter blast was torture," increased their sufferings as they ploughed their way on and on through such depths of snow as they had never seen before. Many died of cold and hunger, many fell grievously sick, and others suffered from snow-blindness and frostbite. But Xenophon led his army on, making his notes of the country through which they were toiling, measuring distances by the day's march, and at last one day when the soldiers were climbing a steep mountain, a cry, growing louder and more joyous every moment, rent the air-- "Thalassa! Thalassa! The sea! The sea!" True enough, on the distant horizon, glittering in the sunlight, was a narrow silver streak of sea--the Black Sea--the goal of all their hopes. The long struggle of five months was over; they could sail home now along the shores of the Black Sea. They had reached the coast near the spot Colchis, where the Argonauts landed to win the Golden Fleece long centuries before. In a work known as the _Anabasis_, Xenophon wrote the adventures of the Ten Thousand Greeks, and no geographical explorer ever recorded his travels through unknown countries more faithfully than did the Greek leader of twenty-three hundred years ago. CHAPTER V ALEXANDER THE GREAT EXPLORES INDIA Still greater light was shed on the size of the world by Alexander the Great on his famous expedition to India, by which he almost doubled the area of the world known to the people of his time. It was just sixty years after Xenophon had made his way right across Asia to the shores of the Black Sea when Alexander resolved to break, if possible, the power of the Persians.