Transcaucasia (trănzˌkôkāˈzhə, –shə, trănsˌ–) [key], transitional region between Europe and Asia, extending from the Greater Caucasus to the Turkish and Iranian borders, between the Black and Caspian seas. It comprises the Republics of Georgia, Armenia, and Azerbaijan. Tbilisi, Bakı, Batumi, Yerevan, and Kutaisi are the major cities. Between the Greater Caucasus in the north and the Lesser Caucasus in the south is the Colchis lowland. The Kura, Rion, Inguri, and Alazan rivers are important for both hydroelectricity and irrigation. The region's natural resources are oil, manganese, copper, clays, and building stones. Manufactures include oil-industry machinery, mining equipment, metal products, automobiles, chemicals, plastics, cotton and silk cloth, and leather footwear. The climate is sub-tropical. The area's chief crops are cotton, grain, sugar beets, sunflowers, tobacco, citrus fruits, tea, and plants for essential oils. Transcaucasia's mineral springs have given rise to numerous health resorts; seaside resorts also abound. The population consists of Georgians, Armenians, Azeris, Assyrians (Christians), Ossets, Abkhas, Talyshin, Kurds, and Tats. An independent federal democratic Transcaucasian republic existed in 1917–18. The federation was dissolved in May, 1918, into the republics of Armenia, Azerbaijan, and Georgia. After the three republics were conquered by the Red Army, the Transcaucasian Soviet Federated Socialist Republic was formed; it joined the USSR in Dec., 1922, becoming one of the four original federated republics. In 1936, Armenia, Azerbaijan, and Georgia were reestablished as separate union republics. In 1991, all three republics seceded from the USSR.