1 107 
Arid Regions of Western China . 
aggravated by the scorching winds which blow throughout the vegetative 
season ; consequently we here meet with an open formation of stunted 
shrubs and herbs more or less specialized to combat the hostile conditions. 
Characteristic shrubs are Sophora viciifolia , Bauhinia densiflora , and 
several other Leguminosae, Clematis Delavayi , Ceratostigma Griffithii , 
species of Pertya , Wikstroema , and so on, none of them rising more than 
two or three feet from the ground. 
A good deal of bare rock is necessarily exposed— indeed viewed from 
any distance these gorges often look quite devoid of life, except for the 
cultivable oases above referred to. 
It is worth noting that the usual structure of river valleys is completely 
reversed in the case of mountain streams entering these arid gorges, and 
this is particularly true where the rock is of limestone. A river as a rule 
flows through ravines and gorges near its source, its valley gradually grow- 
ing broader, till finally the stream is wandering sluggishly across a flat 
flood-plain. Here, however, a stream having its source high up in the 
mountains flows in a comparatively broad valley, winding through alpine 
pastures ; gradually the valley contracts, passing from a typical U-shape in 
cross section to a pronounced V, the stream plunges down a long stairway, 
and finally cuts its way abruptly through a deep gorge to join the main 
river. 
In this case the phenomena of reversed valley structure is emphatically 
a function of the climate apart from the nature and jointing of the rocks. 
I have drawn attention to it because it accounts satisfactorily for the fact 
that, while the main valley presents nothing but a dreary waste of rock, 
scattered amongst which are withered herbs and dwarfed spiny shrubs, the 
side ravines are often densely choked with vegetation, fresh and green. 
This is not due to the fact that the stream itself supplies water to the 
cliffs and precipices of the gorge, which it certainly does not, but to the 
circumstance that the sunlight is kept out nearly all day, and consequently 
the heavy dews in a region of intense radiation are able to supply the 
requisite amount of water, a point I noticed frequently in the Mekong 
valley. Future travellers in these regions may however note that this 
reversed valley structure makes a journey up the Salween or Mekong one 
of the most appalling nightmares possible. 
Though the bigger torrents enter the main valleys through richly 
vegetated gorges, the smaller and more intermittent ones throw out alluvial 
cones, and it is under these circumstances that irrigation by means of 
terracing becomes possible, so that villages occur only where a small stream 
debouches from the mountains. 
Since in South-Eastern Tibet the rain-bearing winds pass over the 
mountains from the west, the valleys as we go east receive less and less 
rain, so that the most abrupt transition from a rainy to an arid climate 
