36 TRANSACTIONS OF THE [Sess. lxxx 



hy CB. There are, of course, hundreds of complicating 

 and modifying factors of which no notice has been taken, 

 and the tangled nature of the mountain ranges with their 

 endless spurs and dividing valleys has been entirely 

 ignored. Nevertheless, I believe that, underlying all the 

 subsidiary details, this fundamental structure can be traced, 

 and that it is readily recognisable on a good physical map 

 of Asia. 



Part of the complicated mountain system in Western 

 China is, of course, easily accounted for by erosion ; and the 

 more irregular the distribution of rainfall, the more tangled 

 the system. Other irregularities are caused by rivers 

 cutting their way back and capturing other rivers — thus 

 the Yangtze, cutting its way westwards, appears to have 

 captured its present headwaters after the parallel ridges 

 had begun to be thrown up, and the same might be true 

 of the Mekong and Salween cutting their way back to the 

 north. Again, the peculiar courses of the four rivers 

 already referred to may be due to shearing in two direc- 

 tions at right angles, as described above — for it is certain 

 that there have been two sets of uplift acting at right 

 angles to one another, probably alternating ; at present it 

 seems that the movement from the west is going on, so 

 that the parallel divides are increasing in altitude as we go 

 westwards, and the ice retreating from those to the east. 



In these rather academic speculations on the geological 

 history of the country, I liave tried to account for the 

 fact of the retreating monsoon by the theory of rain screens, 

 and for the formation of the rain screens by supposing a 

 pressure acting from the west to have pushed up these 

 parallel divides, thus breaking the continuity of an original 

 Sino- Himalayan range, postulated to account for the 

 common alpine flora from the Himalayas to Western China, 

 and giving us the present configuration of the region ; so 

 far as I can see, there is no way of accounting for the 

 Sino- Himalayan flora, except on the supposition of previous 

 continuity. 



We now come to the all-important (|uostion, How far 

 does the theory account for the actual distribution of 

 plants throughout the region, their mutual relationships, 

 the great wealth of liora along the Burma-Yunnan frontier. 



