140 The Wonderful Mekong 
to face it; the loads became loose, the men lost their tempers, 
and I began to regret having ever adventured myself in 
these storm-swept Tibetan uplands. 
This was obviously the rainy side of the plateau, for 
the thick forests of fir extending above 13,000 feet told 
their story as plainly as did the much dissected ridges, the 
curious isolated cones of earth, and the wide-mouthed 
valleys with their fan-shaped loads of detritus. These latter 
were short and broad, forming equilateral triangles, as did 
the valleys from which they had been shot out. Lower 
down, the valley floor was littered with large boulders and 
gravel through which the torrent, now grown to consider- 
able size, cut its way, sometimes flowing in a deep ravine. 
No doubt the heavy summer rainfall of the plateau country 
is due not so much to the rain-bearing winds from the 
S.W., which we have already shown to be stopped by the 
Mekong-Salween divide, but to the local hot winds blowing 
up the Mekong rift at last unloading on the high cold 
plateau the moisture they have gradually accumulated. 
The Mekong gorges are here deeper, narrower, and 
more arid than ever, but this local gale cannot blow day 
after day for several months through all these miles of 
gorges without sooner or later throwing down some moisture. 
A point must be reached at which the pressure becomes 
such that the hot air is forced to rise and spread out, as it 
seems to do north of A-tun-tsi, where the Mekong swings 
away to the north-west and a long arm of the plateau is 
thrust down between it and the Yang-tze. It is natural to 
suppose that the broad expanse of high plateau would 
condense more rain than does the higher but extremely 
narrow ridge further south, and it is this difference of rain- 
fall that we want to account for. 
After condensing most of their moisture on the Mekong- 
Salween divide, the prevailing winds sweep on over the 
open plateau, which for that reason—namely, the exposed 
nature of the country at this high altitude—can support 
nothing but grass-land. By the time the high limestone 
ridge overlooking the Yang-tze is reached there is scarcely 
any moisture left, so that, as usual, barren screes prevail 
above 16,000 feet. 
Thus in the plateau region, starting from the Mekong 
