1915-lG.] BOTANICAL SOCIETY Of" EDINBURGH 33 



iiig more or less north and south. (The general direction 

 of strike throughout the region is about N.N.E. to S.S.W.) 

 The curious fact that the tributary streams of the big 

 rivers often flow parallel to the latter for most of their 

 course, before turning abruptly to enter them, thus sub- 

 dividing the main ridges, and that this tendency is more 

 marked as one goes westwards towards the supposed source 

 of the pressure, seems to me strong evidence in favour of 

 lines of weakness. Thus the parallel ranges come to be 

 more and more closely packed, though reduced in altitude, 

 as one goes westwards ; a fact, however, partly to be attri- 

 buted to increased erosion. It is germane to the present 

 discussion to draw atttention to the tremendous lateral 

 3xtent of the Salween-Irrawaddy divide near the sources 

 of the latter river ; and as the Tibetans say it takes seven 

 or eight days to cross from river to river, the range is 

 probably double or treble in this region. Five or six 

 parallel ranges separate the 'Nmai-hka from the Mali, 

 and a still greater number the Mali from the Brahmaputra. 

 It is sio;nificant that the great mountain ranges of Central 

 and Eastern Asia trend east and west, and that the rivers 

 which break through this gateway to the south begin b}" 

 flowing eastwards. This is particularly true of the Tsangpo 

 or Brahmaputra, which for hundreds of miles flows due east, 

 and in a lesser degree of the Yangtze and Salween. The 

 Tsangpo cuts its way right across the main axis of the 

 Himalayas, while the other two swing round through the 

 great gap and flow due south, the Salween maintaining this 

 course alongside the smaller Mekong, while the Yangtze 

 presently resumes its journey eastwards. Before doing so, 

 however, it makes a remarkable loop, not like the usual 

 S-bend, but more like the letter N upside down, thus \/\. 

 Strangely enough, the same whimsical course is followed b}^ 

 three other rivers in this region : the Yalung, a tributary of 

 the Yangtze further east; the Oui-chu , a tributary of the 

 Salween in Eastern Tibet ; and the Ngawchang-hka, a 

 tributarj^ of the 'Nmai-hka in the south. 



Now imagine an uplift, simple or of fan structure, its 

 long axis trending more or less east and west, subjected to 

 a gradually increasing pressure from one end, the adjacent 

 country having been, as already pointed out, pulled towards 



THAXS. EOT. SOC. EDIN. VOL. XXVII. 3 



