In the mean time, Darius, leading the great body of the army, advanced from the Bosporus by land. The country which the troops thus traversed was Thrace. They met with various adventures as they proceeded, and saw, as the accounts of the expedition state, many strange and marvelous phenomena. They came, for example, to the sources of a very wonderful river, which flows west and south toward the AEgean Sea. The name of the river was the Tearus. It came from thirty-eight springs, all issuing from the same rock, some hot and some cold. The waters of the stream which was produced by the mingling of these fountains were pure, limpid, and delicious, and were possessed of remarkable medicinal properties, being efficacious for the cure of various diseases. Darius was so much pleased with this river, that his army halted to refresh themselves with its waters, and he caused one of his monuments to be erected on the spot, the inscription of which contained not only the usual memorials of the march, but also a tribute to the salubrity of the waters of this magical stream. At one point in the course of the march through Thrace, Darius conceived the idea of varying the construction of his line of monuments by building a cairn. A cairn is a heap of stones, such as is reared in the mountains of Scotland and of Switzerland by the voluntary additions of every passer by, to commemorate a spot marked as the scene of some accident or disaster. As each guide finishes the story of the incident in the hearing of the party which he conducts, each tourist who has listened to it adds his stone to the heap, until the rude structure attains sometimes to a very considerable size. Darius, fixing upon a suitable spot near one of his encampments, commanded every soldier in the army to bring a stone and place it on the pile. A vast mound rose rapidly from these contributions, which, when completed, not only commemorated the march of the army, but denoted, also, by the immense number of the stones entering into the composition of the pile, the countless multitude of soldiers that formed the expedition. There was a story told to Darius, as he was traversing these regions, of a certain king, reigning over some one of the nations that occupied them, who wished to make an enumeration of the inhabitants of his realm. The mode which he adopted was to require every man in his dominions to send him an arrow head. When all the arrow heads were in, the vast collection was counted by the official arithmeticians, and the total of the population was thus attained. The arrow heads were then laid together in a sort of monumental pile. It was, perhaps, this primitive mode of census-taking which suggested to Darius the idea of his cairn.