6 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
also at Dzamtrang in the valley of the Nyang Chu some sixteen 
miles to the south of Gyantse. The smaller intrusive masses are not 
superficially connected with the main granitic core of the crystalline 
zone, but their close genetic relationship to it renders it probable that 
they are merely off-shoots from one and the same magma, ramifying 
along lines of weakness in the folded sedimentary beds. The more 
intricate the ramification, the more complicated will be the structural 
conditions and consequently the orography, and it would seem there- 
fore that the change from the comparative simplicity of the northern 
range of the Himdlaya in their central portion to the complexity in 
the eastern, is due largely to the extensive intrusions of granite along 
lines of weakness, and that the further we go to the east the more 
complex will the conditions become, since we approach nearer to the 
area in which the two great systems of folding, the Himalayan and the 
Tibeto-Burmese, meet ' and in which the structural conditions would 
naturally be expected to be more highly complicated. 
2. Rivers. 
To the north of the crystalline zone lies the Tibetan zone, com- 
posed chiefly of sediments deposited in the Tethys, the Mesozoic sea 
of Eurasia. In Central Tibet, as elsewhere in the Himalaya, these 
sedimentary beds are intensely folded. The longitudinal axes of 
the folds run approximately W.-E., parallel to the Himalayan arc, 
and thus determine the directions of the main drainage lines — a longi- 
tudinal system from west to east and transverse systems from north 
to south and south to north. 
The longitudinal system is represented by the great trough of 
the Tsangpo which, from its source to at least 
thJ'TsangJ'o"^' ^= ^^"^ ^^^^ meridian of Lhasa, follows 
with remarkable accuracy the trend of the 
Himalayan arc. 
At a short distance below Shigatse the river appears to enter a 
' Suess : Das Antlitz der Erde, Vol. I (1892), 590. 
( 127 ) 
