1196 
In the small rocky isle near the mouth of the S* Mahang, peg- 
matites with much dark ov light coloured mica are almost similarly 
constituted as the surrounding granites; the feldspar crystals measuring 
up to ten decimeters, found at the same place, which are beautifully 
graphic-eranitically intérgrown with quartz, have been mentioned above. 
A kind of pegmatitic inter- 
growth of quartz and black 
tourmaline in equal propor- 
tion appears to be a wide- 
spread rock in the granitic 
area,numerous boulders of this 
type being found in the 5" 
Pemandang, a left branch of 
the Rokan Kiri. 
Under the microscope large 
quartz erystals are seen with 
undular extinction or aggre- 
gate polarisation intergrown 
with isolated irregularly sha- 
Fig. 3. Goncretion of one quartz and one ped fragments of tourmaline 
tourmaline crystal. of which a great many extin- 
T =tourmaline. Qu. = quartz. guish at the same time and 
consequently belong to the same crystal. The tourmaline chiefly shows 
brown absorption colours, more bluish colours sometimes occurring 
in the bordering zones; a blue tourmaline is occasionally crystallized 
in small erystals at the periphery of the brown crystals. In these 
pegmatites as yet no tin-ore was found; about the same thing is 
reported by Tosier*) from the granites of the mountains of Doeablas 
in Djambi, which as well as the granite of the Rokan-county are 
closely related to the tin-granites of Banka and Malakka. Numerous 
pneumatolytic veins consisting of quartz and tourmaline are found there. 
THE SURROUNDING SCHISTS. 
The schists which separate the granites along a portion of their 
southwestern limit from the tertiary sandstones and conglomerates, 
we laid bare by denudation in several places on both borders of the 
Rokan Kiri, and they can be excellently observed on the banks of 
the Sei Pakis, which crosses the whole length of the strip of schists 
northwest of the Rokan Kiri. 
1) A. Toren. Voorloopige mededeeling over de geologie der Residentie Djambi. 
Jaarb. v. h. Mijnwezen in Ned. Oost-Indië, 1910; Verhandelingen, p. 20. 
