The double river-systems of the Volga and Kama, the Ob and Irtysh, the Angara and Yenisei, the Lena and Vitim on the Arctic slope, and the Amur and Sungari on the Pacific slope, are instances.
Some of the deposits appear to be of Permian age, but others are probably Jurassic; and they are all included under the general name of the Angara series.
Here again there are no marine beds of Mesozoic or Tertiary age, while plant-bearing deposits belonging to the Angara series are known.
The formation of this and of the other great mountain chains of central Asia resulted in the isolation of portions of the former central sea; and the same forces finally led to the elevation of the whole region and the union of the old continents of Angara and Gondwana.
Lake Baikal receives over 300 streams, mostly short mountain torrents, besides the Upper Angara, which enters its north-east extremity, the Barguzin, on the east, and the Selenga on the south-east.