1198 
Biotitic granites. 
A biotitic granite of the right border of the Rokan Kiri near 
Tanah Dingin contains besides orthoclase some microcline and very 
little acid plagioclase. Quartz is found in large quantities and shows 
the aggregate polarisation or undular extinction commonly occurring 
in the granite quartzes of strongly folded regions. Muscovite only 
occurs in very small quantity in this rock; ore and apatite are both 
present. The two last mentioned minerals are often intergrown with 
biotite. 
Similar biotite granites have been collected on the left border or 
the Rokan Kiri, in the sharp bend downstream from Tandjong 
Medan. Granophyric intergrowths of quartz and feldspar were found 
here in small quantities. 
According to ABENDANON, green augite occurs in granites rich 
in biotite from the upper part of the Se Pemandang. (Cf. “Jaarboek 
van het Mijnwezen” for 1902, p. 138.) 
Two-mica-granites. 
The medium-grained rocks in which the numerous pegmatites of 
the Batoe Kandik occur, belong to this group. They are poor in 
dark constituents and consist chiefly of orthoclase, some microcline 
and acid plagioclase and quartz. Brown as well as green biotite 
occur, often intergrown in one crystal and in alternating layers. 
Small quartz crystals are sometimes poikilitically enclosed by the 
feldspars; the feldspars are slightly sericitized. In these rocks again, 
the quartz shows aggregate polarisation and undular extinction. 
Ore and apatite occur in very small quantities only. 
In the medium-grained rocks with the numerous pegmatites from 
the cliffs near the mouth of the 5% Mahang the percentage of plagio- 
clase has increased, whereas muscovite is found in smaller quantities 
than biotite. Besides orthoclase some microcline is found. The plagio- 
clase has almost the constitution of oligoclase-albite, the quartz 
shows aggregate polarisation, small rounded crystals of quartz are 
sometimes poikilitically enclosed by the feldspars. Some of the feld- 
spars are feebly sericitized, whereas larger muscovite crystals more 
or less in parallel position one to the other are sometimes enclosed. 
Mediuin-grained granites very poor in mica minerals protrude as 
rocks in the bed of the Rokan Kiri near Tandjong Medan. 
Quartzdiorites. 
These rocks contain less quartz and generally more biotite than 
the granites deseribed above. 
