8 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
The Lachen and Lachung are already well known from the 
. . w^ork of Hooker, Blanford and others. They 
Lachen and Lachung. 
rise on opposite sides of the Drongkhya La, the 
Lachung from the southern side and the Lachen from Tso Lhamo 
and the surrounding snow-fields on the north. Both these rivers have 
been described by Hooker, to whose graphic and accurate account 
I have nothing to add. His sketch of the country to the north of 
the Drongkhya La shows admirably the broad open valley through 
which the Lachen flows for the first few miles, before it turns 
at Gyaugang to cut its way through the moraines and talus from 
Chomoyumo and Kangchenjhao. 
Separated from this valley by the comparatively low ridge running 
from Chomoyumo to Pauhunri, is the Yaru plain 
YaruChu. . 
in the centre of which stands Kampa dzong; 
through this meanders the Yaru river, flowing at first approximately 
N.-S. but subsequently turning to the west and flowing out through 
the hills to the west of Mong-go into Nepal. The basin drained by 
this river is of very considerable extent, but there is reason to suppose 
that it was formerly even greater than it is now. High up on the 
hills near Kampa dzong, many hundred feet above the plain, are 
found patches of old river gravels, containing well-rounded boulders, 
many of which are of granite and have apparently been derived from 
Pauhunri and Kangchenjhao. Numerous erratic blocks of granite 
are also found in the valleys and on the slopes to the west of 
Tatsang and can only have been derived from the granite peaks to 
the .south. It appears therefore that the glaciers of Pauhunri and 
Kangchenjhao at one time extended to the north into the Yaru 
plain, and that the upper valley of the Lachen river originally belonged 
to this drainage system but has been subsequently captured from it 
by the gradual cutting back of the head-waters of the Tista. It is 
even possible that the Yaru river itself may have flowed northward to 
join the Tsangpo, but of this we have no direct evidence, and the 
present line of water-parting between the northern and southern 
transverse drainage is the crest of the range which forms the northern 
( 129 ) 
