F. Kingdon Ward — "Land of Deep Corrosions." 151 



An excellent illustration of the almost physical barrier which seems 

 to be stretched across these valleys at the northern limit of rain 

 is sometimes seen, the clouds, from which rain may be falling at the 

 time, stopping short suddenly, and a ribbon of blue sky following 

 the river northwards, though heavy masses of cloud still rest on the 

 mountain-tops immediately above. 



The phenomenon also illustrates the local character of the up-valley 

 winds, for the clouds are entirely unaffected by it, gradually sailing 

 over from the south-west under the influence of the prevailing wind. 



In the Mekong valley just south of the arid region brilliant sun 

 haloc. are frequent in the summer, usually heralding rain ; they are 

 probably caused by clouds which have been blown across from the 

 western range only partially disappearing during their passage over 

 the hot valley, leaving a film through which the sun is seen. The 



Fig. 4. Lake occupying a rock-basin at 16,000 feet on the Mekong-Yangtze Divide. 



passage of clouds over the arid region on the other hand is often 

 quite invisible, the rain drenching the western divide and later the 

 eastern without affecting the intervening valley. 



The effects of this abrupt change of climate in the Salween valley 

 are various. In the first place the appearance of the valley changes 

 from a typical U in cross section to a pronounced V, and at the same 

 time the spurs change their direction from one of more or less 

 parallelism to the main axis, to a direction more or less at right angles 

 to it, thus forming as it were supporting buttresses. 



Another change is to be noted in the tributary valleys which, in 

 the arid region, exhibit to a very marked degree the ' reversed ' type 

 of structure, whereas in the more rainy regions to the south this 

 structure, though sometimes apparent, is not nearly so conspicuous. 

 [By a ' reversed ' valley I mean a valley which begins in the mountains 



