6o HAYDEN : GEOLOGY OF THE PROVINCES OF TSANG AND 0. 
The Kyi Chu rock, on the other hand, is essentially a hornblende- 
albite-granite, and is composed of quartz, orthoclase, albite, microcline, 
perthite, oligoclase, hornblende, biotite, apatite, epidote, sphene and 
magnetite. The plagioclase is very abundant, hornblende and biotite 
are always present in considerable quantities, while the sphene is a 
conspicuous and almost essential constituent of the rock. In the 
neighbourhood of Lhasa there are two varieties of the granite, a 
coarse-grained form with much quartz and having a specific gravity 
of 2'65, and a fine-grained, less acid type, in which sphene and the 
ferro-magnesian minerals are very conspicuous and raise the specific 
gravity to 2"]0. This rock differs in several points from the acid 
variety. It is especially characterised by the presence of various 
lime-bearing minerals including sphene, epidote and calcite, of 
which the first two are particularly conspicuous both in the hand- 
specimen and under the microscope. The presence of such a 
marked calcareous element in the granite amounting to nearly 4*7 
per cent, of CaO is at first sight surprising, since the basic 
felspar is neither large enough in quantity nor far enough advanced 
towards alteration to have yielded an appreciable amount of epidote 
and calcite. Nor apparently have either of these minerals arisen 
from the decomposition of the hornblende ; this may have 
occurred in certain instances, where the calcite and epidote are 
found in association with hornblende, but in other cases they are 
associated neither with hornblende nor with plagioclase, but appear 
to have crystallised out independently among pre-existing individuals 
of quartz and orthoclase; thus in the case shown on PI. 14, fig. 4, the 
calcite is found surrounding and enclosing fragments of a single 
crystal of orthoclase, the various parts of each mineral being, res- 
pectively, in optical continuity ; the epidote also shows a similar 
habit. At the same time the epidote and calcite frequently occur in 
association with one another and with the sphene, and all tliree ap- 
pear to ha^^e arisen at one and the same stage in the life-history of 
the rock. A simple explanation of their origin seems to lie in the 
absorption, by the granite, of lime from the rocks into which it has 
( '81 ) 
