The Ustun-tagh, although like the twin ranges of the Western Kuen-lun, it forms the outermost escarpment on the north of the Tibetan plateau, would appear, in the opinion of the most compe tent judges, to be only a branch or subsidiary range of the main range of the Kuen-lun. It is not, however, a single chain, but consists of two parallel ranges, and in the east of three, and even north-east of Tsaidam of four, being flanked throughout by several subsidiary chains, spurs and offshoots. Beyond, it swells into the vast massif of Anambaruin-ula, which is traversed by at least three minor parallel chains. Farther east it once more contracts to two main ranges, the more southerly being the Humboldt Range (crossed by a pass at 13,20o ft.). This branch is probably con tinued in the South Kukunor Range. The northern branch merges eastwards into the Nanshan Mountains. The passes in the Lower Ustun-tagh range from altitudes of 10,15o to 10,70o ft., and in the Upper Ustun-tagh of 11,77o to 15,68o ft., one pass beside the Charkhlik-su being only 9,66o ft. high. The general elevation of the twin border ridge decreases towards the east, but there exists a striking difference between the crests of the Ustun-tagh and those of the ranges of the Tibetan plateau. The Ustun-tagh moun tains are severely weathered and consist of hard, bare and barren rock, weathered into sharp peaks and pinnacles. The ranges on the Tibetan plateau on the other hand are of softer rock and have rounded outlines. In the former regions aeolian action removes the products of disintegration and with arid conditions prevailing the mountains are left bare. In Tibet, precipitation is heavy and uni formly distributed so that disintegration is likewise heavy and the mountain sides are covered with great masses of scree material. The twin ranges of the Ustun-tagh are fairly equivalent in point of magnitude and regularity; but while the Lower Range, on the north, sensibly decreases in altitude towards the east, the Upper Range maintains its general altitude in a remarkable way.
The next parallel range on the south, the Akato-tagh, and the valley which separates it from the Ustun-tagh, are equally arid and waterless. The valley, known by the general name of Kakir, mean ing a "hard, dry, sterile expanse of clay," is chequered with shal low self-contained basins of the usual type and has remarkably gentle slopes up to the mountains. Its surface slopes from alti tudes of 10,i oo to 10,60o ft. in the west, where is lake Uzunshor (9,650 ft.) to 9,400 ft. in the east. Farther east it continues to the Anambaruin-ula and the flat basin of Sartang, a north extension of Tsaidam. Like the Ustun-tagh it stretches towards the east-north east, and appears to be built up of granite and schists, but its crest is greatly denuded. The slopes on both sides are very gentle, but that on the south is eight to ten times as long as that on the north. In the east the range is narrow, and dies away on the edge of the Tsaidam depression ; but in the west it swells out into the lofty mass of the Ilve-chimen or Shia-manglay, which is capped with perpetual snow. This part is crossed by the pass of Chopur alik (16,16o ft.), but farther east the passes lie at altitudes of 13,38o to 10,520 ft. The latitudinal valley south of the Akato tagh, the Chimen-tagh, slopes for the most part eastwards, from 12,500 ft. down to the shallow salt lake of Gass-kul (9,305 ft.).
The Chimen-tagh is identical in its western parts with the Piazlik-tagh and in the east with the Tsaidam chain, and it is prob ably continued westwards by Moscow Range or the Achik-tagh, running north of the Achik-kol and connecting on the west with the Tokuz-davan. The Chimen-tagh rises into imposing summits, rounded or pyramidal, which in winter are capped with snow. Its slopes are not so arid as those of the Akato-tagh and the Ustun tagh. Snow falls all the year round on the Chimen-tagh, and water is abundant everywhere. The southern slope of the range
is gentle but short, the northern slope long and steep. Grass is able to grow, and animal life is more abundant. The range is crossed by passes at 13,97o, 13,23o and 13,76o ft., and the Piazlik tagh by a pass at an altitude of 13,64o ft.
The next important range, still going south, is the Kalta a'aghan (Columbus Range). A short secondary range, the Ara tagh, rises along the middle (ara=middle) of the valley between the Chimen-tagh and the Kalta-alaghan. It is of lower elevation than them both and it dies away towards the west, the valleys on each side of it meeting round it to form one broad valley, with an altitude of 11,790 to 13,725 ft. The Ara-tagh is crossed by a pass at an altitude of '4,345 ft. In the Kalta-alaghan, which is the cul urinating range of this part of the Kuen-lun, and is over-topped by towering, snow-clad peaks, the passes are at high altitudes, viz. 14,470, 14,430 and 14,190 ft., while the pass of Avraz davan is at 15,700 ft. This range appears to be linked on to the Tokuz-davan by the Muzluk-tagh, in which are passes at 16,87o and 15,45o ft.
Immediately south of the Kalta-alaghan comes a relatively deep depression, the Kum-kol valley. It is crossed transversely by a water-divide which separates the basin of the twin-lakes of Kum kol (12,70o ft.) from the basin of Tsaidam, some 3,50o ft. lower. The floor of the valley consequently slopes away in both direc tions, like the Chimen valley. Not far from the Kum-kol lakes there is a drift-sand area, though the dunes are stationary. The upper lake of Kum-kol (Chon-kum-kol) (52,73o ft.), which con tains fresh water, is of small area (8 sq.m.) and shallow (13 ft.); but the lower lake (Ayak-kum-kol) (12,685 ft.), which is salt, is much bigger (283 sq.m.) and 64 and 79 ft. in depth. Farther west, lying between the Muzluk-tagh and the Arka-tagh, is the lake of Achik-kol ft.), 164 m. broad.
The next great parallel range is the lofty Arka-tagh, which is continued eastward in the Marco Polo Range (altitude 15,750- 16,25o ft.) and Gurbu-naiji Mountains. The Arka-tagh is the true backbone of the Kuen-lun system, and in Central Asia is exceeded in elevation only by the Tang-la, of the Karakorum Mountains. The Arka-tagh is the actual border-range of the Tibetan plateau; to the south none of the lofty parallel ranges which ridge Tibet seems to have any connection with the Kuen-lun system. Of great length, the Arka-tagh, a mountain-system rather than a range, varies greatly in configuration, sometimes exhibiting a sharp main crest, with several lower flanking ranges, and sometimes numerous parallel crests of nearly uniform altitude. Amongst these one can distinguish four predominant ranges, of which the second from the north is probably the principal range, though the fourth is the highest. These lofty mountains are covered with sand and powdery, finely sifted disintegrated material and where hard rock does crop out on the surface, it is excessively weathered. The cul minating summits are frequently crowned with ice caps which shape themselves into a flat cap, from which there are offshoots at intervals; nowhere do there exist any of the long, glacier tongues which are so characteristic of the Alps. The view south from the heights of the Kalta-alaghan is the picture of a chaos of mountain chains, ridges, crests, peaks, spurs and detached masses. Im mediately north of the Arka-tagh the country is studded with three or four conspicuous and detached mountain masses, all capped with snow and some of them carrying small glaciers. The next succeeding parallel range, the Koko-shili, which is con tinued eastwards by the Baien Kara Mts. between the upper head streams of the Hwang-ho and the Yangtsze-kiang, belongs oro graphically to the plateau of Tibet.