1194 
From the first rocks in situ on the left bank of the Rokan Kiri, 
in the sharp bend downstream of Tandjong Medan, and from the 
rocks projecting there from the right bank specimens of dioritic 
rocks were collected consistmg chiefly of plagioclase with quartz 
and biotite, the further constituents being amphibole, titanite, apatite, 
some chlorite and very little ore. . 
The plagioclase has -almost the same composition as andesine, 
some more acidic mixtures (oligoclase-andesine) also occurring, and 
a feebly zonar structure with repeated alternations of zones more 
acidic and more basic being rather common. Exceptionally the 
plagioclase was found to be partly surrounded by a narrow margin 
of granophyric texture in which the feldspar is not polysynthetically 
twinned. In slides of the symmetrical zone of the plagioclase this 
last feldspar shows a straight extinction, this fact pointing to ortho- 
clase. The quartz shows aggregate polarisation, the lamellae of the 
plagioclases being bent. The biotite with very small axial angle is 
as a rule fresh, exceptionally some alteration into chlorite occurs. 
Green amphibole is found in small quantity, titanite occurs in grains 
and in more or less rounded crystals, and, as well as the apatite, is 
often enclosed by biotite. On the other hand small biotite crystals 
in places are enclosed by a large crystal of titanite. 
Granitic dykes in the diorites. 
At a distance of about 2 m. from each other on the left bank 
of the Rokan Kiri in the sharp bend down Tandjong Medan two 
dykes were found cutting the quartz diorites. These medium to fine- 
grained rocks, light coloured, consist largely of orthoclase (and micro- 
cline) and quartz. Acid plagioclases are found in small quantity, 
the further constituents being: biotite, apatite, and some green 
chlorite. Ore is almost totally absent. The chlorite is sometimes 
found alternating in layers with biotite in the flakes of the latter 
mineral. An intergrowth of kalifeldspar with quartz is remarkable 
in which numerous small rounded or almost idiomorphie crystals 
of quartz enclosed by the feldspar extinguish at the same time. 
Granophyrie intergrowths, in which the quartz is irregularly shaped, 
also oceur in small quantities. 
One of the granitic dykes in the quartz diorites on the other bank 
of the Rokan Kiri consists entirely of a mixture of quartz and feld- 
spar, some traces only of biotite and chlorite occurring. 
(rneissoid granites. 
In a left branch of the Pakis river, near the kampong Pakis, 
