IGNEOUS ROCKS. 
57 
reached and that in the Tertiary system of southern Tibet we have 
a series analogous to the older Tertiaries — Ihe lower Ranikot — of 
western Sind. 
CHAPTER VIII. 
IGNEOUS ROCKS. 
Both acid and basic igneous rocks are fairly numerous throughout 
the area visited by the mission. The former consist chiefly of granite 
and the latter of an extensive series of basic dyke-rocks, including 
diorite, augite norite, epidiorite and serpentine. In addition to these 
a number of pebbles of volcanic rocks allied to dacite were found in 
the gravels of the Tsangpo between Kampa-partsi and Chaksam ; 
these were not found in situ but most probably occur in the Tsangpo 
valley between Kampa-partsi and Shigatse. 
In the valley of the Nyang Chu basic intrusions are numerous and 
have been noticed at various points in the hills on either side of the 
river. To the south of Gyantse several dykes are seen cutting 
through the Jurassic slates ; one of these forms a ridge at about seven 
miles to the south-south-west of the town and consists of much 
decomposed lath-shaped plagioclase, with large quantities of chlorite, 
ilmenite, leucoxene and apatite. The most striking feature of this 
rock is the great abundance of the apatite, the freshness of which 
contrasts strongly with the advanced state of alteration of all the other 
constituents. 
At Tsechen and again at Drongtse, there is a coarser greyish- 
green rock, composed of plagioclase, — almost completely replaced by 
saussuritic decomposition products — with chlorite, some quartz, 
ilmenite, much leucoxene, and a little calcite and actinolite ; this 
rock appears to have been originally a diorite, and probably marks a 
stage of decomposition rather more advanced than that of the basic 
intrusions at Gupshi, at the junction of the Ralung and Nyiru 
rivers. This latter rock, although much altered, is still recognisable as 
( 178 ) 
