170 Reviews — Dr. Molengraqfjt" s Central Borneo. 



Semitau and other localities. Petrographically tliey are similar to^ 

 crystalline schists of Archaean age, but it is possible that they are 

 only sediments altered by granite intrusions. 



Next, above the doubtful schists, there is a great thickness of 

 phyllitic clay-slate with a silky lustre, alternating with beds of 

 sandstone, greywacke, and quartzite. The strata are nearly vertical. 

 No fossils have been found, and its age is therefore unknown, though 

 very possibly it is Palgeozoic. It has been named the "Old Slate 

 Formation." The Upper Kapoewas range, a typical mountain chain 

 much worn down by erosion, which runs nearly east and west and 

 forms the boundary between West Borneo and Sarawak, and between 

 West and East Borneo for a distance of 225 kilometres, is mainly 

 built up of these slaty strata. On the south side of the range the 

 beds are abruptly cut off by a great fault. 



The next younger formation, known as the " Danau Formation," is 

 composed of highly tilted and contorted beds of chert, jasper, and 

 hornstone, quartzite, clay-slate, sandstone, diabase, diabase-tuff, and 

 porphyrite, which have been brought down to the level of the " Old 

 Slate Formation " by a fault. The beds have an east and west 

 strike, and they have been traced from the Lake district for a distance 

 of 280 kilometres, quite into East Borneo. The chert, jasper, and 

 hornstone beds are of organic origin, and are mainly composed of 

 Kadiolaria. In the Upper Kapoewas region their thickness is 

 estimated at 100 metres, and they are regarded by the author as deep- 

 sea deposits. The beds of diabase and diabase-tuff, which alternate 

 with the chert and hornstone, are attributed to submarine and 

 volcanic eruptions. Some of the tuff beds likewise contain Radiolaria. 

 This formation is of pre-Cretaceous age, and the Radiolarian beds are 

 probably Jurassic. 



A system of moderately tilted and folded beds of marly and sandy 

 deposits succeeds the Danau formation. Along the river Seberoewang 

 the marls contain OrhitoUna concava, which indicates that they are of 

 Cenomanian (Upper Greensand) age. The deposits were probably 

 laid down near a coast, and a portion of Central Borneo must then 

 have been dry land. The rocks occur in various localities, and extend 

 to the south of the Madi plateau. 



The oldest Tertiary deposits in Central Borneo have not been met 

 with in situ ; they are only known from boulders of grit, containing 

 Nummulites and Orbitoides, which occur in the valleys of the rivers 

 Embaloeh and Tekelan ; they may be regarded as of Oligocene age. 



An enormous area is covered by a formation named the Old 

 Tertiary or Melawi Group, which consists of horizontal beds of sand- 

 stone, quartzitic sandstone, and claystone, with intercalated seams of 

 coal. Molluscan shells occur in these beds in places ; according to 

 Krause and Martin they are brackish-water forms, which probably 

 lived in estuaries. At the time of their deposition the whole of 

 Central Borneo, with the exception of the Upper Kapoewas range, 

 was submerged beneath the sea. The greater part of the Madi 

 plateau and the Schwaner range, which separates West from South 

 Borneo, are formed of these rocks. 



