150 F. Kingdon Ward — "Land of Deep Corrosions." 



to check the higli rainfall of the Sal ween valley, and consequently we 

 here pass abruptly from a region of dense forest to a region of semi- 

 desert; the Mekong-Sal ween divide is even higher, so that while 

 gathering to itself whatever remains of the moisture in the air, and 

 boasting an equally heavy rainfall, the Mekong valley or gorge 

 beyond is, if possible, more desiccated than the Salween valley. 

 But after crossing two high ranges the winds have now little 

 moisture left, so that not only is the Yangtze valley arid but the 

 Mekong-Yangtze divide also (Plate V) ; thus it stands in marked 

 contrast to the other two rain-drenched ridges. 



To return to the valleys. The combined effect of high wall-like 

 mountains and lack of rain causes a local wind to blow in each valley 

 throughout the summer as the heated air rises and cold air sweeps 

 down from the snows immediately above to take its place, and since 



Fig. 3. An oasis in the arid region, Mekong Kiver Valley, north of Yangtze. 



the gorges get narrower towards the north the partial vacuum is 

 more complete in this direction, and consequently the wind almost 

 invariably blows from south to north up the valleys. It begins 

 about ten o'clock in the morning, reaching its maximum intensity 

 towards evening, and dies away about midnight or rather earlier. 

 Meanwhile the rainfall has been reduced to a few inches a year, 

 and from a distance the valley has the appearance of a desert, except 

 where an alluvial cone, by permitting a little terracing and irrigation, 

 forms an oasis (Fig. 3). 



The most abrupt transition from luxuriant vegetation to semi-desert 

 occurs on the Salween (Fig. 2), for the next two valleys are, to 

 a certain extent, deprived of a copious rainfall even south of the 

 snowy ranges, which we have called rain-screens ; hence the transition, 

 thou"h obvious, is not so remarkable. 



