CARBONIFEROUS SYSTEM. 45 
At the head of the Lipak river, the uppermost carboniferous lime- 
stones are overlain by alternating beds of shale and 
Po series. 
quartzite. The thickness of the series is variable : 
it appears first on the right side of the Lipak river, where the total 
thickness of the shale and quartzite is small, but to the north, towards 
Spiti it increases, and in the ranges on either side of the Spiti river 
below Po it is not less than 2,000 feet. Near Sumra and Lari the 
lower beds of the series consist of shale and quartzite, the former 
predominating. The quartzites are yellowish, brownish or white, and 
often resemble the Muth quartzite. The shales are usually black, but 
near Sumra are reddish- brown and schistose ; this is due to contact- 
metamorphism, numerous dykes and sheets of basic igneous rock 
(amphibolite and altered dolerite) occurrinij among the sedimentary beds. 
In the neighbourhood of the intrusions, the shales have been converted 
into hard slates and pyritous and garnetiferous mica schists. One of 
these dykes runs for several miles along the hillside between Sumra 
and the pass (Sumra La or Shdlkar Li) leading into Kanaur, and is seen 
again in the hills on the left side of the Spiti river, opposite Sumra; it 
is of very uniform thickness and is inclined at a low angle to the 
bedding-planes of the shale and quartzite: it thus frequently resembles, 
and is at times, a sheet, and it is probable that it was mistaken by 
Stoliczka for a contemporaneous trap flow, for he states that " between 
Changrizang and Po beds of greenstone 
Supposed contem- ^ -1 n .1 1 ,1 
poraneous trap. [occur] all through the series 
not in veins, but in regular beds between the other 
rocks." A careful examination was made by the present writer of all 
the igneous rocks seen between Po and Changrizang, and in every case 
they have been found to be intrusive, and although; at times, they 
take the form of sheets, yet if followed for any distance, they invariably 
cut across the bedding-planes of the sedimentary beds. These ap- 
parent sheets can be seen in the neighbourhoods of Sumra and Lari, 
wliile near Po — about ? mile north-east of the village — the path 
' Memoirs, G. S. 1., vol. V, p. 20. 
( 45 ) 
