22 HAYDEN: GEOLOGV OF THE PROVINCES OF TSANG AND 0. 
not only the whole of the Trias but possibly one or more of the 
Palaeozoic systems. It is very unfortunate that, owing to the condi- 
tions prevailing at the time of my visit to this area, it was im- 
possible to examine these beds in detail, but it is to be hoped that at 
some future date they may be rendered accessible and an exhaustive 
study may be made of this small patch of sedimentary rocks. The 
importance of their bearing on Himalayan geology can hardly be 
exaggerated, for at present they constitute the only outcrop of 
undoubted pre-Jurassic sedimentary beds of Tibetan facies known 
to occur between Byans and the Subansiri river to the north of 
Assam.i 
The total area covered by the Dothak series is very considerable, 
and probably extends throughout the ranges lying between Dothak 
and the Phari plain on the one side, and the valley of Punakha in 
Bhutan on the other. 
The only fossils noticed in the series were found in the section 
near Dothak, already described, and on a lime- 
Trias (?) near Phari. . ' ' 
stone ridge on the south side of the Tremo La. 
In the latter locality the fossils consisted of fragments of bivalves, but 
none of those found in the course of a rapid examination of the beds 
were determinable ; since, however, out of three localities visited 
under circumstances which precluded more than a very hurried 
examination, two afforded fragmentary fossils from the Dothak series 
and the third determinable brachiopods from the beds immediately 
overlying that series, it is reasonable to suppose that a more careful 
survey of the hills to the south of Phari would offer a fair prospect 
of a successful determination of their stratigraphical position. 
The gradual thinning out of the Trias from a total thickness of over 
4,000 feet in Spiti on the west to 2,500 feet in Byans on the east is 
one of the most interesting points recently established in Himalayan 
' The presence of Permo-Carboniferous beds in the valley of the Subansiri 
has been inferred from boulders of fossiliferous limestone found near the mouth 
of that river by Mr, J. M. Maclaren (Rec. Geol. Surv. Ind., XXXI, p. 186; 
XXXII, p. 190). 
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