1197 
So far as they do not show any traces — or to a slight degree 
only — of contactmetamorphism, they are of small mineralogical 
importance ; the contact-rocks only will be mentioned underneath. 
Many of the rocks contain calcite in varying quantity, and the 
series of rocks examined includes various gradations between 
limestone, sericite schist, and chert or quartzitie schist free from calcite 
and sericite. Limestones are e.g. exposed for a great distance in the 
St Mangis, a small right branch of the Sei Pakis, about halfway 
between the kampong Pakis and the Rokan Kiri. Those are partly 
White semi-crystalline limestones, partly they are very dark coloured 
or rich in strongly pigmented veins. Farther upstream in the river 
Mangis these rocks are covered by the tertiary sandstones and 
conglomerates. 
Transitions of these rocks into phyllites rich in calcite by intermediance 
of rocks with little quartz and sericite in a calcitic matrix are found ; 
whereas by an increase of the percentage of quartz and mica, 
transitions into the calcium-phyllites are formed. Calcium-plhyllites 
with sericite schists are e.g. exposed in the banks of the S* Pakis 
near its mouth, and higher upstream in the river similar rocks 
alternating with rocks richer in calcite and with. limestones are 
repeatedly exposed. The sericite schists are sometimes macrosco- 
pically dense rocks, sometimes they are crystalline and strongly 
schistose. The latter often contain tourmaline, whereas also varieties 
rich in pyrite are found. | 
The tourmaline is often more or less idiomorphous in the prismatic 
zone, sometimes the crystals being divided into various irregular, 
simultaneously extinguishing parts, separated by the quartz-sericite- 
mixture. 
The quartz shows aggregate polarisation and undular extinction, 
kataklastic structures often occurring. 
Fine-granular and strongly schistose two-mica-schists containing 
much mica which are found in the left bank of the Sei Pakis, 
contain some tourmaline and pyrite, and moreover in lenticular portions 
which are poor in mica or free from mica some feldspar was found 
in small quantity between the quartz crystals. This fact in connection 
with the high percentage of biotite and with the facts to be mentioned 
underneath, renders it probable that these rocks have already to a 
certain degree been affected by contact-metamorphism, 
- Finally the occurrence of brecciated rocks may be mentioned con- 
taining fragments of sericite schists cemented by a fine aggregate of 
quartz grains. In these sericite schists black layers strongly pigmented 
alternate with those containing little pigment. 
