IGNEOUS ROCKS. 
63 
has already been pointed out that it is almost certainly not older than 
upper Cretaceous. The mineralogical characters of the granite differ 
so markedly from those of the typical biotite-granite of the Himalaya, 
that one is at first tempted to regard the two as belonging to separate 
petrographical provinces, but an analysis of the difference shows that 
it consists almost entirely in the preponderance in the Kyi Chu rock 
of lime and lime-bearing silicates, which as stated above may be 
explained by modification of the granite by the rocks into which it 
was intruded. 
The numerous pebbles of dacite and other volcanic rocks found 
in the Tsangpo gravels between Kampa-partsi and Chaksam, appear 
to be genetically related to the Kyi Chu granite and may very 
possibly be extrusive portions of the same magma. At the same 
time the diabases and serpentines described above may equally 
represent yet another phase in the process of differentiation. 
Both the dacites and the diabases are remarkable for the advanced 
state of alteration of many of their constituents, such as the felspars 
and the ferro-magnesian silicates, and are consequently rich in secon- 
dary minerals, such as epidote, chlorite, calcite, serpentine and 
leucoxene. In marked contrast to this is the comparative freshness 
of the constituents of the hornblende-diorite of Chaksam. This rock, 
however, as already stated, occurs in the immediate neighbourhood of 
the granite; it is coarse and granitoid in structure and was therefore 
presumably formed at a considerable depth below the surface, in what 
Van HiseHerms the "zone of anamorphism," whereas the highly 
altered rocks were injected as dykes — possibly even extruded as 
surface flows — among the higher layers of the crust, termed by the 
same author the " zone of katamorphism," which he regards as the seat 
of such secondary changes as hydration, oxidation and carbonation. 
The mere accident of position, or relative depth below the surface, 
would thus suffice to account for the varying degrees of alteration of 
different parts of the same magma. 
' C. R. Van Hise : A Treatise on metamorphism, U, S. Geol, Survey Mono- 
graphs, XLVII (1904). 
( 184 ) 
