B. B. Verbeek — Geology of Sumatra. 479 



pyroxene-porphyries. They have a dark-coloured matrix, in which 

 are imbedded crystals of faint-white plagioclase, green pyroxene, and 

 magnetite. The magnetic iron-ore shows partly octahedral forms 

 and large crystals ; and it occurs copiously in excessively small grains 

 throughout the matrix, which is coloured dark by it ; and it is also 

 found inclosed in the crystals of pyroxene. The crystals and grains 

 of magnetite, even in the thinnest microscopical slices, are always 

 opaque. 



5. The Tertiary deposits, which follow next, are to be sub-divided 

 into four groups. 



a. Breccias, conglomerates, arkoses (sandstones, derived from decomposed syenite, 



granite, and quartz-porphyry), and marl-slates; the last contain remains of 

 Fishes and Plants. This lower part of the Tertiary formation is called the 

 Breccia-stage, or Breccia-group. The thickness differs greatly at various lo- 

 calities. 



b. Sandstones, with clays and coals. Some Fishes and Plants. The thickness of this 



portion, called the Sandstone-group, varies from 300 to 500 metres. 



c. Marl-sandstones. Shells, etc.. The thickness of this group is at least 500 metres, 



and at some places probably much more. 



d. Limestone, with Corals, Shells, etc. , and abounding with Orbitoides. The thick- 



ness is 120 metres. 



5 a. The breccias and conglomerates contain fragments of the 

 several older rocks, — syenite, granite, quartz -porphyry, Fusulina- 

 limestone, schists, etc. 



The arlcose is a sandstone whose substances have been derived from 

 syenite, and partly also from quartz-porphyxy ; the beds of coarsest 

 grain contain balls of hard syenite ; the beds of finer grain alternate 

 with beds of the most remarkable rock of this group, namely, the 

 marl-slate. 



These marl-slates have proved to be fossiliferous at several locali- 

 ties on the Rivers Sipang, 1 Malakoetan, Sangkareivang, Loera Ge- 

 dang, and in the neighbourhood of the village of Telaweh ; they 

 contain Fishes and Plants. Between the marl-slates occur very thin 

 beds of hard shale ; and it is remarkable that the Fishes are always 

 imbedded at the bottom of the marl-slates. It is thus probable that 

 the Fishes lived in the water which deposited the shales, but that 

 the great quantity of lime contained in the water which deposited 

 the marl-slates was unfavourable to their existence. 



The marl-slate was deposited in the neighbourhood of the old 

 coasts as a littoral deposit, and received the land Plants from the 

 coast. 



5 b. The sandstones of this group are composed of quartz-grains 

 cemented by an argillaceous paste. The colour is yellowish or brown. 

 This is the Sumatran Coal-formation. The beds of coal vary in num- 

 ber and thickness at different localities ; and they are generally near 

 the base of the series. The Oembilien Coal-field contains about 200 

 millions of tons (1 ton=1000 kilograms). In the northern part of 

 this coal-field, seven or eight coal-seams are known ; in the southern 

 portion, the so-called Soengei-Doerian Coal-field, there are only three 



1 The pronunciation of the Dutch vowels is the same as in German, except the oe, 

 which is the German u ; thus the Dutch a is pronounced as the English a in «re, the 

 e as the e in latter; the e as a in male, the i as e in he, and the oe as oo in good. 



