96 hayden: geology of spitu 
lowest undoubtedly sedimentary rocks into the gneiss, show that it 
must either represent an altered sedimentary series, which would then 
probably be the lower silurian quartzite, or be an old intrusive rock, 
in which case the gneissose beds between it and the sedimentaries 
would represent a contact zone. 
The composition of a typical specimen of the augen gneiss is as 
follows : — 
Si O2 ........ 75*09 per cent. 
<o\} 'S-^? 
Ca O '90 „ 
MgO -54 » 
K3O 3-8 „ 
Naj O 2'i „ 
HjO -39 .. 
10179 
Specific gravity 2*65 
This, it will be seen, is the composition of a true granite, and it is 
not improbable that the so-called gneiss is in reality only the remains 
of an old crushed-out granite laccolite. A very similar rock is found 
in the Chandra valley, on the road from Spiti to Kulu ; but in this 
case it is undoubtedly a foliated granite. 
The final decision of this question must be left till opportunities 
arise for a detailed survey of this part of Laddkh ; it is probable that 
an examination of the area lying to the north-west of Tso Moriri 
would throw some light on the relations of the augen gneiss to the 
sedimentary rocks as well as to the Rupshu granite. 
Igneous rocks, both acid and basic, are common throughout South- 
ern Rupshu ; a few of these will be dealt with in the following 
chapter. 
( 96 ) 
