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synclinal belt G is located, which has now been folded and converted 
into land for the greater part. The belts G, and G, gradually pass 
one into the other. The seaward strip (@, is richer in lime- 
stones than G,, which lies more landward, as has been shown by 
Rurren. In G, no, volcanic action has taken place. According to 
Rutten the deposits in the Tertiary geosyncline of Kast-Borneo attain 
a thickness of about 5500 m. and comprise the entire Miocene, 
perhaps even a part of the Oligocene, and the Pliocene. Beds of 
lignite as well as beds of petroleum occur in this geosyncline. 
Rurten, from the differences in facies of the deposits, and before 
him VerBeeK concluded that the N.S.-shoreline of Kutei existed already 
in the Old-Miocene, and that at that time the Strait of Macassar had 
already been formed as a more or less deep trough. The oil-field of East- 
Borneo thus has been developed in a geosyncline, which lay between 
Borneo, a part of Sunda Land being the denudation-area from which it 
derived its sediments, and Macassar Strait being the adjoining oceanic 
area. This conception is accepted only provisionally, and some stress 
may be laid on the point that the geological position of Celebes relative 
to this geosyncline and to the Strait of Maccassar is not explained by it. 
II. Sumatra and Java. 
The geosynelinal belt G, to which the present oil-tields are con- 
fined (see map fig. 2) lies along the north-east and the eastcoast 
of Sumatra and the north coast of Java, bordering the Java Sea, the 
South China Sea and Malacca Strait. It is of Tertiary, Neogene age 
and the belt G,, which contains petroleum and lignite in many 
places, is now slightly folded and the major portion has become 
land. Towards the ocean follows the adjoining belt G,, which had 
already been folded and converted into a mountain-chain, whilst 
subsidence still continued in the portion G, of the geosyncline, and 
the process of sedimentation was still in progress there. The moun- 
tains of Sumatra, which I will designate here by the collective name 
of Barissan Mountains, represent one geanticline, and the row of the 
Mentawei-islands and others west of Sumatra represent another 
geanticline in these folded mountain-chains. More to the west follows 
the sea S, in this case the Indian Ocean. In the belt G, intense and 
prolonged volcanie activity has taken place in Sumatra as well as 
in Java; this activity commenced as early as the Old-Miocene. In 
the Miocene already volcanoes rose, presumably as a row of islands 
above the sea-level, for from that time andesitic material is found 
in the geosynclinal deposits of the belt G,. But, where has one to 
look for the primary area of denudation £ from which these geo- 
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