The succession of ranges from the deserts of Takla-makan and Gobi to Tibet rise in steps, each range being higher than the range to the north of it. The difference in altitude between the lowest, most northerly range, the Lower Ustun-tagh, and the most south erly of the Arka-tagh ranges amounts to nearly 7,500 ft. With one exception, namely the climb out of the Kum-kol valley to the Arka-tagh, the first three steps are individually the biggest. When the altitudes of the intermont latitudinal valleys are compared, the significance orographically of the Chimen valley and of the Kum-kol valley is strikingly emphasized. These ten parallel ranges of the middle Kuen-lun system may be grouped in three divisions —( ) the more strictly border ranges of the Upper and Lower Ustun-tagh and the Akato-tagh; (2) the three ranges of Chimen tagh, Ara-tagh and Kalta-alaghan, which may be considered as a transitional system between the foregoing and the third division; (3) the Arka-tagh, which constitute the elevated rampart of the Tibetan plateau proper.
The Nan-shan Highlands overlook Tsaidam on the north-east. They consist of parallel mountain ranges all running from the north-west to the south-east. Broad, flat, longitudinal valleys, at altitudes of 12,000 to 14,00o ft. (9,00o to i o,000 at the south western border) and dotted with lakes (Koko-nor, 9,97o ft.; Khara-nor, 13,285 ft.), fill up the space between these mountain ranges. In the south-east are the highlands of the Chinese prov ince of Kansuh ; near the northward bend of the Hwang-ho, the escarpments by which the Great Khingan and the In-shan ranges are continued, and by which the Mongolian plateau steps down to the lowlands of China ; on the north-east, the Mongolian plateau (average altitude, 4,00o ft.), i.e., in the Ala-shan; and on the north-west a border range, the Da-sue-shan, a continuation of the Ustun-tagh, with passes at 12,20o-13,000 ft., and is pierced by
several rivers flowing west to Lake Khara-nor.
On the south-west the Nan-shan mountains consist of short irregular chains, separated by broad plains, dotted with lakes (altitude from 8,800-9,000 ft.). Next, narrow ranges intervene between this lower border terrace and the higher terrace 13,50o ft.). The first mountain range on this higher terrace is Ritter's range, covered in part with extensive snow-fields. There are passes at both ends of this massif at 15,990 ft. and 14,68o ft. The next range is the Humboldt range, which runs from the Ustun-tagh to about N.
Generally speaking, the Nan-shan highlands (12,00o to 14,000 ft.) are intersected by wild, stony and partly snow-clad mountains, towering another 4,000 to 7,000 ft. above its surface, and arranged in narrow parallel chains. The chains of mountains are from 8 to 17 m. wide, seldom as much as 35, while broad, flat valleys between them attain widths of 20 to 27 miles. The passes are at 12,000 to 14,000 ft., and the peaks reach 18,000 to 20,000 ft. in the west, while in the east they are 2,000 ft. lower.
enumeration of the works published before 1890, will be found in Wegener's Versuch einer Orographie des Kuen lun (Marburg, 1891). Of the books published since 1890 the most important are Sven Hedin's Scientific Results of a Journey in Central Asia, 1899-19o2 (Stockholm, 1905-1907, 6 vols.), with map of Tibet on scale of ni,o00,000; Trans-Himalaya (3 vols., 1920), and Journeys in Tibet (Geog. Journ., 1909) ; H. H. P. Deasy's In Tibet and Chinese Turkestan (London, 1901), with a good map; Sir F. Young husband, Peking to Lhasa (1925) ; St. G. R. Littledale's "A Journey across Tibet," in Geog. Journ. (May 1896) ; E. Suess, The Face of the Earth (5 vols., English trans., Oxford, 5904-24) ; the Izvestia of the Russian Geog. Soc. and Geog. Journal, both passim.