As to the picturesque Bureya mountains on the Amur, the forest-clothed Sikhota-alin on the Pacific, and the volcanic chains of Kamchatka, they belong to quite another orographical world; they are the border-ridges of the terraces by which the great plateau-belt descends to the depths of the Pacific Ocean. It is owing to these leading orographical features--divined by Carl Ritter, but only within the present day revealed by geographical research--that so many of the great rivers of the old continent are comprised within the limits of the Russian empire. Taking rise on the plateau-belt, or in its Alpine outskirts, they flow first, like the upper Rhone and Rhine, along high longitudinal valleys formerly filled up with great lakes; next they find their way through the rocky walls; and finally they enter the lowlands, where they become navigable, and, describing great curves to avoid here and there the minor plateaus and hilly tracts, they bring into water-communication with one another places thousands of miles apart. The double river-systems of the Volga and Kama, the Obi and Irtish, the Angara and Yenisei, the Lena and Vitim on the Arctic slope, the Amur and Sungari on the Pacific slope, are instances. They were the true channels of Russian colonization. A broad depression--the Aral-Caspian desert--has arisen where the plateau-belt has reached its greatest height and suddenly changes its direction from a north-western into a north-eastern one; this desert is now filled only to a small extent by the salt waters of the Caspian, Aral and Balkash inland seas; but it bears unmistakable traces of having been during Post-Pliocene times an immense inland basin. There the Volga, the Ural, the Sir Daria, and the Oxus discharge their waters without reaching the ocean, but continue to bring life to the rapidly drying Transcaspian Steppes, or connect by their river network, as the Volga does, the most remote parts of European Russia.