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- National Geographic Atlas of the World (7th ed.). Washington, DC: National Geographic. 1999. ISBN 0-7922-7528-4. "Europe" (pp. 68-9); "Asia" (pp. 90-1): "A commonly accepted division between Asia and Europe ... is formed by the Ural Mountains, Ural River, Caspian Sea, Caucasus Mountains, and the Black Sea with its outlets, the Bosporus and Dardanelles."