478 JR. D. Verbeek — Geology of Sumatra. 



1. The oldest rocks of this part of Sumatra are granites, granite- 

 syenites, and syenites, in several modifications. There are granites 

 which contain only felspar, quartz, and mica ; but a great part of 

 them contain also amphibole. The syenites contain, beside orthoclase 

 and amphibole, almost always quartz and some mica ; but the 

 granites have more quartz than the syenites. A great part of the 

 rocks of this group may be best called "syenite-granite," or "granite- 

 syenite," as they stand in composition between granite and syenite. 

 There seems to be no difference in the age of these rocks, as there are 

 syenites which regularly pass into granites. The felspar of the gran- 

 ites and syenites is partly orthoclase, partly plagioclase, which shows 

 its triclinic nature by the fine varicolored laminated structure, when 

 examined under the microscope with polarized light. The quartz 

 contains always a great number of " fluid-cavities." 



2. Next in order follow sedimentary rocks, which are probably of 

 either Carboniferous or Permian age, as they contain Fusulince, which 

 are only met with in rocks belonging to the Carboniferous and Per- 

 mian periods. Both Professor T. Rupert Jones and Professor H. B. 

 Geinitz, to whom I submitted some of these fossils, determined them 

 as Fnsulino3 ; but the Encrinital stems which occur in our Pusulina- 

 limestone have, as Professor Geinitz informed me, a younger appear- 

 ance, reminding him of the Triassic Encrinus Cassianus, Laube. 



This oldest sedimentary formation of Sumatra can be dmded into 

 two parts. The lower portion consists of clay-slates, with auriferous 

 quartz-veins, marl-slates, and siliceous schists ; the upper part consists 

 only of limestone, with some small beds of schists. This limestone 

 contains the Fusulinai ; but these fossils also occur in some limestone 

 beds which are found between the schists of the lower part. The 

 schists and the limestones are conformable one with another. They 

 are widely spread all over Sumatra, and form great mountain-ranges 

 in the Highlands ; and are often accompanied by greenstones, which 

 will be described hereafter. 



3. Quartz-porphyries are probably younger than the schist- and 

 limestone-formation ; some quartz-porphyries, at least, inclose frag- 

 ments of schists ; but it is not yet proved that all the quartz-por- 

 phyries of the Highlands are of the same age. 



These rocks always show, when examined with the microscope, an 

 amorphous and so-called " felsitic " matrix, which is not resolved by 

 the highest magnifying powers into crystalline grains. In this paste 

 are imbedded crystals and grains of quartz (with many "fluid-cavi- 

 ties"), crystals of felspar (orthoclase and some oligoclase), and some 

 fragmentary, green, dichroitic crystals, which belong to amphibole. 



4. Greenstones. These rocks, as stated above, are often associated 

 with the older schists and limestones, which are dislocated and heaved 

 up by them, in such a manner that portions of those rocks lie some- 

 times as islands upon the greenstones. The age of these rocks is not 

 exactly known, but it is sufficiently proved that their eruption took 

 place before the Tertiary Period, and that they consequently are not 

 to be confounded with the greenstone -trachytes of Hungary. The 

 Sumatran greenstones are pyroxenic rocks, partly diabases, partly 



