1192 
VerBeeEK *) describes numerous quartz diorites, which probably form 
irregular patches in the granites and some veins of quartz diorite 
in granitite. In the granitic area of the Rokan county granitic veins 
occur in quartz-dioritic rocks and thus a part of tne granites must 
be younger than the quartz diorites. 
FACIES OF THE GRANITES. 
The rocks of the granitic area which were examined contain 
two-mica granites, biotite granites and their transitions into quartz- 
diorites. The quartz-diorites examined are rich in biotite and contain 
a green amphibole in small quantities. 
Porphyritic vocks with very large feldsparphenocrists are very 
numerous, just as in the Malakka granites. Often the phenocrists 
show a more or less distinct parallel arrangement, and the porphyritie 
rocks sometimes occur in alternating layers with the normal ones. 
Rocks with gneissoid structure also occur. 
Pegmatitic facies are very numerous. Along both borders of the 
Rokan Kiri and in the numerous side-rivers of the Rokan Kiri 
running through the granitic area, their outcrops are repeatedly 
visible as more or less irregularly defined masses or as veins running 
regularly for some distance; as a rule, these rocks are rich in 
tourmaline. In the rocks Batoe Kandik projecting on the right side 
of the Rokan Kiri pegmatites are found containing very much 
biotite, their crystals measuring sometimes several cm’, and for the 
rest consisting chiefly of feldspar, quartz and tourmaline. Dykes of 
quartz tourmaline rocks free from feldspar also occur here; the 
rock which intersects these pegmatites is a medium grained granite 
without phenocrists. The rock which protudes at the mouth of 
the St Mahang in the middle of the Rokan Kiri shows a great 
variety of rocks. Along with medium-grained granites one finds 
here very many pegmatitic segregations which for the greater part 
consist of feldspar, quartz, tourmaline with dark or light-coloured 
mica, Whereas patches with the structure of graphic granite protrude 
as knobs from the surrounding rock. 
Dykes which are more acid and contain fewer dark minerals than 
the rocks they intersect were found in several places. To quote one 
instance dykes of light-coloured biotite granite on various points cut 
through quartzdiorites on both borders of the Rokan Kiri. 
1) R. D. M. VerseeK, Topographische en geologische beschrijving van een ge- 
deelte van Sumatra’s Westkust, 1883, p. 220. 
