585 



character; often the schistose structure has entirely disappeared, 

 whereas "felspathisation" as a characteristic contact-phenomenon is 

 missing. Tourmaline often occurs here in such large quantities at the 

 contact of the granites, that for the greater part tiie rocks consist 

 óf this mineral. The tilted more or less schistose limestones with 

 graduations into sandstones, qtiarlzites and hornfeis of the Goenoeng 

 Soeligi, on the border of tiie iiill countries of Siak and the subdi- 

 vision Boven Karapar of the government Sumatra's Westkust, are 

 mainly covered to the North-East by sedimentary terrane. In the 

 beds of the S'^' Lau and S'' Rambei, which have their sources on 

 the G» Soeligi, however, similar rocks are repeatedly uncovered. 

 Veins of quartz are numerous in these rocks. Near the top of the 

 Goenoeng Soeligi, N.W. and N.N.W. strikes with N.E. dips of 

 65' and 70° were measured, whereas to the North-East side of the 

 Gt' Soeligi in the S"' Lau, up the river from Kota Renah, N.W. 

 strikes and N.E. dips of 50° to 60° were found. 



The occurrence of detached pieces and of weathering-products of 

 granite in the neighbourhood of Kota Renah (hill countries of Siak) 

 has already been mentioned by Evervvijn') (1864) whereas on a sketch- 

 map of RoLKER ^) granite is indicated in the last right branch of the 

 S*-'' Kalemboi, a right branch of the S"' Lau. Further |)ebbles of 

 hornfeis from the S"' Lau have been collected and described by 

 Verbeek '). The "big, rounded diorite-stones" too, which Evkrwun 

 found near and in tlie kampong Kota Renah, agree, judging from 

 his microscopical description, with some of our hornfeis. 



The occurrence of cassiterite, although not met with in the material 

 as yet examined, as a component of rocks in situ in the neighbour- 

 hood of Kota Renah is very pi'obably in connection with the character 

 and distribution of alluvial tin-ore in the S<"' Lau and its side-rivers. 

 Coarse and fine ore occur mi,\cdly, llie ore being often very sliarji- 

 edged and sometimes intergrown with quartz, whereas it was not 

 found in the upper part of the S'' Lau. 



The examined granites -are rocks containing touruialine and are 

 free from biotite, tliev were collected in tiie right bank of the last 



^) R. EvEKWiJN. Verslag van eeu onderzoekiiigsreis in hel rijk van Siak. Jaarb. 

 V. h. Mijnwezen v. N. 0. -Indie 1874, and N.itiiurk. Tijdschr. v Ned. Indie, vol. 

 XXIX, 1867 



-) Charles M. Rolker. The alluvial tin-deposits of Siak, vSumatra. Trans. 

 Americ. Institute of Mining Engineers, vol. XX (1891), p. 50. 



3) R. D. M. Verbeek. TopograplusfI.e en Geologische Bestlirijving van een 

 Gedeelte van Sumatra's Westkusl, Batavia 18S3, p. 610, 612. 



38* 



