64 HAYDEN: GEOLOGY OF THE PROVINCES OF TSANG AND U. 
The presence of serpentine among the basic rocks intrusive in the 
sedimentary systems of India and Burma and its almost complete 
absence from the dunites and other olivine-bearing rocks found in the 
old crystalline complex of the Indian peninsula, led Mr. Holland to 
the conclusion that extensive serpentinisation is probably due to 
submarine agencies.^ If we apply this theory to the serpentines of 
Central Tibet, their age will be limited by that of the most recent 
marine sediments deposited in the old Fibetan sea, and as no stratified 
rocks of marine origin younger than eocene have yet been found, it is 
reasonable to assume that the age of the basic intrusives is not less 
than upper eocene. This is a further argument in favour of correlating 
these rocks with the basic dykes and eocene volcanic series of Ladak 
and Kumaon. 
CHAPTER IX. 
ECONOMIC GEOLOGY. 
The area visited during the recent expedition is strikingly poor in 
minerals of economic value, the only one found 
Gold. 
in situ being gold, which was obtained in very 
small quantities from the coarse gravel beds of the Tsangpo ; these 
were panned at various points near Chaksam, but the largest yield 
was only at tlie rate of "28 grain of gold per ton of gravel. The con- 
centrates were found to contain, in addition to much magnetite and 
zircon, a small quantity of rutile, tourmaline, hercynite, and monazite 
(see Appendix A). 
Persistent rumours of the occurrence of coal both in the Nyang 
Chu valley, near Penang, and at Lhasa proved 
Coal, 
to be groundless. In the former case it is highly 
probable that the supposed coal is the serpentine occurring at 
' T. H. Holland : Bvit. Association Report (1898), 868 : Geol. Mag., Dec. IV, 
Vol. VI (1899). 30, 540. 
( 185 ) 
