1518 
of quartz, brown and green mica in very numerous flakes and long 
cross-sections, plagioclase, opaque felspars and iron ores. The 
laminated granite-apophyses alternating with the felsparhornfels near 
the contact of Rokan, were not found here. 
Contucts of the Siboumboun mountains. 
In the geologically and petrographically very interesting Siboumboun 
mountains, to the east of the lake of Singkarah, the contact of 
granitic rocks and of sediments described by Verserk *) as carboni- 
ferous, appears clearly, especially so in the river Timbuloun, where 
it forms a small waterfall, partly over contact-metamorphic limestone, 
partly over the adjoining granites, opposite the Bukit-Tambang. 
The granite contains dark-coloured inclusions; they are often sharp- 
edged, and their number increases near the contact. In close contact 
with the granites various rocks occur, being partly massive rocks of 
garnet, partly rocks containing much epidote, or apparently not very 
crystalline grey rocks without macroscopically visible contact-minerals 
and with veins of rocks rich in garnet or epidote. The plane of 
contact is sometimes very irregular. In the rocks containing much 
epidote and in the rocks which apparently are not very crystalline 
calcite-crystals of large size enclosing other minerals, sometimes are 
recognizable from lustrous cleavage-planes. 
A few meters from the contact the character of the metamorphic 
rocks is less varying. First, we see a zone of some meters thick- 
ness of a white erystalline limestone with many brownisb-red 
crystals of garnet bands which are very rich in garnet, alternate 
with bands which ‘are very poor in them; farther from the contact 
massive garnet rocks are found, still farther white crystalline lime- 
stones appear, containing again garnet crystals, which occur in different 
layers with more or less frequency and which emerge as warts on 
the weathered surface. 
In the neighbourhood, garnet-rocks are found in several places 
in close contact with granites; and crystalline limestones with 
or without contact-minerals very often oecur, the contact with 
garnet-rocks being e.g. clearly exposed further down the river on 
the right wall of the river Timbuloun, close up the river from the 
left tributary Nibong. The garnet-rocks are often impregnated with 
copper- and iron ores. 
Elsewhere in Sumatra the copper ores appear to be bound to 
1) R. D. M. Verpeek. Le. p. 242, 249. Id. Geologische Beschrijving van 
het Siboemboengebergte. Jaarb. Mynw. 1876. Il. p. 51. 
