20 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
composed of biotite gneiss with intrusive bands of schorl granite— 
and run thence through the mountain ranges to the north of Bhutan. 
1, The Khongbu series. 
The bay formed by this bend in the boundary of the crystalline 
zone contains the only stratified rocks of pre -Jurassic age encountered 
during the recent expedition. The oldest of these occur in the lower 
part of the Khongbu valley, forming the high ridges on either side of 
Talung and extending as far as the upper reaches of the Ammo Chu 
On the north and south and probably on the west ^ they are in direct 
contact with the granite, but on the east they abut against a younger 
sedimentary series from which they are separated by an important fault 
which can be traced from Dothak almost to the Tang La and has 
determined the course of the upper part of the Ammo Chu. On the 
west of this fault are the Khongbu beds, consisting of slate and 
schist with flaggy calcareous bands, often crystalline. These rocks 
are all apparently devoid of organic remains and quite distinct 
from, and probably much older than, anything seen on the east of the 
fault. They are less highly metamorphosed and less disturbed than 
the paraschists and paragneisses of the crystalline zone, but very 
decidedly more so than the neighbouring rocks of Dothak and the 
Phari plain. 
In the preliminary note published in the Records (XXXII, pt. 2), 
the Khongbu beds were referred doubtfully to the Palaeozoic group, 
but subsequent detailed consideration of their characters points rather 
to a correlation with the unfossiliferous pre-Cambrian rocks of the 
more westerly parts of the Himalaya. 
2. The Dothak series. 
To the east of the Dothak-Tang La fault, on the left bank of the 
Ammo Chu, a distinctly younger series of sedimentary rocks extends 
from the northern boundary of the Chumbi granite throughout the 
' Their western boundary, which appears to be on the range separating the 
Lachung and Khongbu valleys, could not be visited. 
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