a 
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porphyritic rocks were found showing a distinet parallel structure 
and containing only few dark constituents. They are poor in plagio- 
clase, microcline occurring in varying quantity with orthoelase. The 
feldspars only occur as phenocrists, as a rule rounded and showing 
more or less irregular contours; they show traces of sericitization and 
sometimes enclose rather large crystals of colourless mica. The biotite 
sometimes contains ore in numerous disseminated minute crystals 
and often it is partly chloritized. 
Pyrite occurs in rather numerous small crystals in some of these 
rocks. Feldspar and quartz are sometimes granophyrically intergrown. 
The crystals of the groundmass are often found enclosed by the 
phenocrists of feldspar and the matrix sometimes penetrates into the 
phenocrisis. The often occurring irregular extinctions of the quartzes 
point to pressure after the crystallization of the rocks. 
Pegmatites. 
Some provisional facts will be given here on the constitution of 
the very numerous pegmatites. The pegmatites rich in biotite, from 
the Batoe Kandik, consist chiefly of plagioclase, orthoclase, quartz 
with undular extinction and biotite, tourmaline and colourless 
mica also occurring. The plagioclase has almost the constitution of 
oligoclase, whereas more acid mixtures approaching to oligoclase- 
albite, also occur. As a rule, the tourmaline shows absorption colours 
of brownish hue, more bluish colours sometimes being found round 
the brown colours as a bordering zone of varying breadth and 
occasionally missing; no sharp distinction exists between these varieties. 
A dyke of a width of about 2 metres with N. N. W. strike, consisting 
of pegmatite containing tourmaline and occasionally much biotite, 
in which numerous small erystals of reddish brown garnet are 
macroscopically visible, over a short distance appears outcropping 
in small rocks, which, the level of the water being low, are visible 
on the left bank of the Rokan Kiri near Tandjong Medan. They 
are rich in acid plagioclase, orthoclase and quartz. Several of 
the plagioclases proved on examination to be acid oligoclase-albite, 
more basic mixtures, however, also occurring. The feldspars some- 
times are to some extent sericitized. The crystals of the garnet are 
idiomorphous and microscopically colourless. The tourmaline is usually 
not idiomorphous in the prismatic zone, the brown colour of the crystals 
is often less deep in the central parties than in the bordering zone, 
more blue colours moreover occurring in the central parts. Finally, 
the occurrence of zircon in small idiomorphous crystals with pyramidal 
limit outlines must be mentioned. 
