South of the trans-Himalaya lie the deep valley lines of the Indus, Sutlej and Brahmaputra, so this range forms the water shed between an area of internal drainage to the north and a region of the sources of great rivers to the south ; it is note worthy that the three great rivers mentioned make their way through the Himalaya to the south, and many feeders of the Brahmaputra, Ganges and Indus have also penetrated into the Himalaya. Between Bunji, in Gilgit, and the Darel district the bed of the Indus is about 915 metres above sea-level, while the crests on either side of the gorge rise to 6,000 metres, or 20,000 feet. The Brahmaputra way through is as striking, for the river reaches down to the 1 o,000ft. contour only as it begins to work around the base of Nameha Batwa, 25,445ft. high.
The long line of the Upper Indus and Upper Brahmaputra is the most marked feature of southern Tibet, and the Upper Sutlej follows a closely related line between them. Some tributaries of the Brahmaputra join the main river in such a way as to suggest that it once flowed westwards, and that its present course is a result of capture. Near the source of the Sutlej are the two sacred lakes, Manasarowar and Rakas-tal, which communicate by a channel of water after heavy rain.
The snow line is usually, but not quite always, higher on the north than in the south, because of the much heavier precipita tion on the south; on this latter side it is said to run near the 5,000 metre (16,400f t.) contour. Near Leh it is at 5,300-5,800 metres, and in the Karakorum at 5,600-5,900 metres. The glaciers formerly reached down to 3,300-3,60o metres in the Spiti region, and to 2,100 metres in the Upper Indus region. It is thus highly probable that during the maximal ice phases the whole of the great highland was glaciated, with no doubt some lines of crest projecting above the ice-sheets. The Pleistocene deposits are enormously thick in many places, an estimate of 3,000ft. having been made for this in the upper Sutlej valley.
A remarkable economic feature is the almost universal dis tribution of gold through Tibet. The gold-digging is referred to in somewhat mythical terms by Herodotus. Every river which rises in Tibet washes down sands impregnated with gold, and it has been proved that this gold is not the product of intervening strata, but must have existed primarily in crystalline rocks of the main axes of upheaval. In western Tibet the gold mines of Jalung have been worked since 1875.
Iron is found in eastern Tibet in the form of pyrites, and is rudely smelted locally. Salt, soda, potash and borax exist in
abundance in the western lake regions (Chang-t'ang).
In July the Tarim basin has average temperatures varying from 68° to as high as 90° F. in different places. On the Tibetan highland figures are lower and are correlated with elevation ; thus, at an elevation of 3,00o metres the temperature in July in north Tibet is known for one or two stations and is about 59°, for a station at 4,000 metres it is less than 50°, and for the general area of Chang-t'ang, with a height of 5,000 metres, it is probably about 40°, or even less. But the daily variations are extreme.
There is more snow in the winter, towards the west, and more rain, in the summer, towards the south-east. The total fall of rain and snow over most of Tibet, except the south-east and patches of the trans-Himalaya, is about 8in. per annum.