4 HAYDEN : GEOLOGY OF THE PROVINCES OF TSANG AND t). 
oriented ridges, intersected by broad valleys and enclosing open 
plains and extensive lakes, and is not definitely separated from the 
southern chain of high peaks, its relation to which is somewhat 
analogous to that of the lower Himalaya to the same range. 
Hence in the area dealt with in the present memoir there is only 
one continuous trough which persists throughout the whole length of 
Central Tibet : this is the valley of the Tsangpo, which divides the 
Central Himalaya from the so-called Tibetan ranges, termed by 
Trelawney Saunders the Gangri mountains and by Hodgson the 
Nyenchentangla^ range. 
On the west, along the northern frontier of Nepal, a second trough 
appears to be fairly well marked and continuous, but disappears in 
Central I'ibet, and can be traced only in a series of more or less 
isolated depressions, such as the Yaru plain and the basin of Kala 
Tso, lying amongst the complicated system of ranges which here 
replace the northern range of the Central Himalaya. 
The irregularity of the orographic conditions of this area is, as 
might be expected, reflected in the stratigraphy; still the threefold 
sub-division into (a) Tibetan, (d) lower Himalayan and crystalline and 
{c) sub-Himalayan zones is for the most part as clearly marked 
between Bengal and Lhasa as it is between the Dun and Hundes or 
between Kalka and Ladak. 
The present memoir deals exclusively with the two inner zones, 
the lower Himalayan and crystalline and the Tibetan. The former 
of these extends throughout Sikkim and Bhutan and consists chiefly 
of granite penetrating a series of schists, crystalline limestones 
' Gang-ri is merely a generic term meaning " snowy peak " and its use by 
the Tibetans is analogous to that of the term " tsang-po " for any large river. 
Nyen-chen-tang-la — ? " N6-jin-thang-lha " (A^o-/j« = mountain deities) — is also 
the name given by Littlrdale [Geo^r. yo;<»'"., VII (1896), 467] for the snowy 
range to the south of Nam Tso (Tengri Nur) culminating in the peak named 
by him "Charemaru" (? Tsa-rz), the height of which he states to be 24,153 feet. 
This is possibly the peak visible from the hills above Gyantse and also from 
the Pem-po-go La; its position was fixed from the hills to the north of Lhasa 
by Major Ryder and the height determined at 23,250 feet. 
( ) 
