PHYSICAL FEATURES. 7 
series of deep gorges cut through granite and possibly various volcanic 
rocks, pebbles of which are numerous amongst the river gravels 
further down : at Chapra the valley rapidly widens and soon becomes 
from two to four miles broad with numerous river channels and old 
gravel banks ; sand dunes are seen at Chaksam where the sand is 
piled against the western side of the hill which stands out prominently 
in the middle of the valley.^ 
Throughout this part of its course the river runs parallel to the 
strike of the Jurassic slates, but further to the east its relation to the 
strike of the sedimentary beds is still only a matter of conjecture 
and at the point at which it bends to the south to cut its way through 
the mountains, the stratigraphical conditions are, as already stated, 
probably highly complicated. It has been suggested to me by 
Mr. .R. D. Oldham that the latter part of its course may afford an 
example of the phenomenon of capture so common in the Himalayan 
rivers, and that originally the Tsangpo may have continued on- 
wards further to the east, but this and many other geological and 
geographical problems of absorbing interest must await solution until 
the unknown country between the Tsangpo and Batang has been 
visited, here the Himalayan system meets the N.-S. system of the 
Burmese and eastern Tibetan folds, and here too it should be possible 
to determine the relationship of the sediments laid down in the 
Eurasian Mesozoic sea to the older Palaeozoic and metamorphic beds 
found by Szechenyi and Loczy in the ranges of Eastern Tibet and 
Western China.' 
The transverse drainage system comprises several large rivers, 
including the Lachen and Lachung (which 
Transverse system. _ 
unite at Tsiintang to form the Tista), the 
Arun (or Yaru) Chu, the Ammo Chu, the Nyang Chu and the 
Kyi Chu. 
' These are entirely wind-formed and show the direction of the prevailing winds 
to be westerly. 
' Wiss. Ergebnisse der Retse der Grafen Bela Szechenyi in Ostasien ; Bd. 
I (1893). 
( 128 ) 
