IGNEOUS ROCKS. 
59 
advanced state of alteration, as evidenced by the presence of secondary 
minerals such as epidote, leucoxcne, calcite and serpentine, and in this 
respect they present a striking contrast to a mass of basic rock, not 
yet referred to, which is intrusive in the Jurassic slates of the Tsangpo 
valley. This rock forms a high and conspicuous hill on the right 
bank of the river at Chaksam and is essentially a coarse-grained 
hornblende-diorite. Under the microscope, it is found to be com- 
posed of plagioclase felspar, partly oligoclase and partly a more basic 
variety approaching labradorite, with some orthoclase and large 
quantities of a brownish-green hornblende occasionally showing 
idiomorphic outlines. There are also a few^ irregular patches of 
hornblende containing a colourless mineral with high refractive index 
and high double refraction, possibly pyroxene which has been 
partially altered to secondary hornblende. In addition to these, there 
is a considerable quantity of biotite, chlorite and magnetite and a little 
sphene ; the chlorite has probably been derived from the alteration of 
'hornblende and biotite. Quartz is present, but in very small quantity. 
The felspar is often beautifully zoned and, as a rule, fresh and little 
altered, in this respect differing very markedly from the saussuri- 
tised felspars of the diabases : indeed, with the exception of the 
chlorite and the small amount of (?) secondary hornblende, the 
Tock is almost free from alteration. It is associated with and prob- 
ably genetically related to the granite of the Kyi Cha valley, 
which is found on the opposite bank of the 
Kyi Chu granite. . 
Tsangpo and runs thence in a broad band along 
the valley of the former river up to and beyond Lhasa. The relation- 
ship of the two rocks is further borne out by the character of the 
granite, which differs markedly from both the typical foliated biotite- 
granite and the muscovite-schorl-granite of the Himalayan crystalline 
zone. The latter varieties as seen in Sikkim and Chumbi differ in 
no respect from the similar granites of the more westerly parts of 
the Himalaya, and, in the upper reaches of the Lachen river resemble 
those of the Sutlej valley in having beryl as one of the constituents 
of the pegmatite, 
( 180 ) 
