The Land of Deep Corrosions 267 
river continues to grind its way between dry bare cliffs. It 
is only for a stretch of about fifty miles south of Yangtsa, 
which may be called the rain-belt, that the Mekong gorges 
support anything approaching forest, and even the luxuriant 
vegetation occupying the gullies of the middle Mekong 
owes its existence, not to a more copious rainfall, but to the 
heavy night dews in a region of intense radiation, which 
are here protected from being immediately lapped up as 
soon as the sun gets into the valley. But as we are now 
wandering somewhat beyond the Land of Deep Corrosions, 
it will be unnecessary to pursue this subject further. 
The rain-belt immediately succeeding the arid region is 
however something of a puzzle. Were it continued south- 
wards, the valley receiving a heavier and heavier rainfall as 
the dividing ridges to the west became lower, there would 
be no difficulty in understanding it. But it is not, for, as 
already stated, a second arid region begins outside the 
limits of the country immediately under discussion, and the 
term ‘arid region’ is henceforth confined to the intensely 
dry valleys lying between the snow-clad ridges soon destined 
to lose themselves in Tibet. 
The effect of this scanty rainfall in the arid region, 
which can hardly exceed five inches annually, is greatly 
aggravated by the local winds, which throughout the summer 
blow up all three valleys with the regularity of the trade- 
winds, setting in soon after mid-day and blowing themselves 
out before midnight. This desiccating wind is due to the 
cold air pouring down from the high mountains immediately 
overhead to fill the partial vacuum caused by the intense 
heating of the narrow valley during the day; and the fact 
that it is almost invariably an up-valley wind may be ascribed 
to the fact that the gorges contract more and more as we 
proceed northwards, so that the vacuum becomes more and 
more complete in this direction. 
I have travelled in the arid regions of both the Salween 
and Mekong valleys during the summer months beneath 
a winding ribbon of blue sky which followed the course of 
the river as a canopy might, and watched the clouds 
thickening on the mountains immediately to east and west 
till their summits were hidden in blinding rain-storms. 
Once at Yang-tsa on the Mekong, which almost exactly 
