F. Kingdon Ward — "Land of Deep Corrosions." 153 



certainly retreating, their bottle-nosed snouts being already some 

 distance from tlie terminal moraines. Other evidence is derived from 

 the 'hanging' valleys whicb descend immediately from the water- 

 shed and their peculiar ' tread and riser' structure, the valley floor 

 ascending in a series of long abrupt steps to higher levels, each 

 platform being the receptacle for a small lake occupying a rock-basin, 

 one of which is shown in the photograph (Fig. 4, p. 151). Some- 

 times as many as five such lakes may occur in one valley, but usually 

 there are fewer. Finally, we have the huge accumulations of angular 

 rock fragments at the heads of the cirques, some of which at least are 

 probably parts of old moraines. Of perched blocks and striae, however, 

 I saw no sign, though I do not attach much importance to that, for 

 the climate is now all against their preservation. 



Now it is evident that if the Mekong- Yangtze divide received as 

 heavy a rainfall as does the Mekong-Salween divide there would be 

 glaciers on the former where none now exist, so that the disappearance 

 of the glaciers may be ascribed to a diminution of the rainfall. Why, 

 then, has the rainfall become less ? Evidently because it has been 

 gradually cut off by the ranges to the west thrusting themselves up 

 higher and higher till at last they form effective rain-screens. Nor is 

 this all. Aneroid readings taken at different points in the beds of all 

 three rivers show that the Yangtze flows at an average level of about 

 a thousand feet above the Mekong, and the latter about the same height 

 above the Sal ween, indicating a general western tilt of the country 

 as it were, which within recent geological times has probably been 

 in a condition of considerable instability ; nor, indeed, are earthquakes 

 rare, slight tremors being, I believe, frequent, which may help to 

 account for the number of landslips, while hot springs issue from the 

 base of every range. Such a tilt might easily cause a cracking of the 

 crust, suggesting that these rivers really break through the rim of 

 the Tibetan plateau along lines of weakness (fissures) or faults ; but 

 without believing for a moment that they have themselves carved out 

 the deep gutters where they now flow, there is evidence, particularly 

 in the Mekong gorges, for a very considerable amount of spade-work 

 — for instance, pot-holes and ancient river cliffs now many feet above 

 the highest flood-mark. 



The crumpling and tilting of the rocks shows how the country has 

 been pounded this way and that, but the general appearance points to 

 a lateral pressure acting from the west (such as might be occasioned 

 by a sli'iht sliding motion of the main Himalayan axis eastwards), 

 squeezing the belt of country up against an unyielding barrier in 

 western China (supplied by the main east and west axis of uplift, 

 separating the two great river systems of China), thus gradually 

 raising the Mekong-Salween divide to its present considerable 

 elevation, and so cutting off the rain supply of the older easternmost 

 ridge — that remarkable freak of nature, the Mekong-Yangtze divide. 



[The Editor is greatly indebted to the Syndics of the University 

 Press, Cambridge, for permission to use the illustrations in this paper, 

 which are reproduced from Mr. F. Kingdon Ward's forthcoming work 

 entitled The Land of the Blue Poppy : Travels of a Naturalist in Tibet. ^ 



