59(1 



few larger quartz-crystals without inclusions being found in the fine- 

 granular mixture. At the eastern contact of this intrusion, or very 

 near to it, even muscovite bearing biotiteschists occurs, in which the 

 schistose structure has been preserved. 



The detached pieces of liornfels already described by Vkkbkk.k, 

 which are found very frequently in the neighbourhood of Kola Renah 

 (similar roclis being met with by me even in the upper stream of the 

 S"' Lau as rocks in situ) prove the great extension of rocks similar 

 to those of the quartz-biolite zone which hitherto have been examined 

 by us only near the contact. They often contain green aniphibole. 

 The other constituents are quartz, biotite, tourmaline, titanite, ilmenite, 

 calcite and pyrite. 



Again, numerous |)ebbles of rocks similar to the quartz-tourmaline- 

 zone, were found in the riveis Lau and Pinggir, proving that 

 these rocks occur also elsewhere ami of more considerable thickness. 

 Of the latter rocks, some with narrow veins of quartz were micro- 

 scopically examined. 



The veins of quartz often — and chietiy in the marginal zone 

 — contain tourmaline, and are sometimes rich in muscovite. Occa- 

 sionally in the marginal zone larger tourmaline-crystals (of sometimes 

 several mm.'s length) are deposed more or less perpendicular to the 

 plane of contact. As a rule, these tourmaline-crystals have a zonar 

 strnotiire. just like the tourmaline of the granites, and contrary to 

 the tourmaline of the adjoining rocks, which most times has a 

 brown colour. Bluey and brown varieties can both occur as a 

 marginal zone, a repeated alternation of differently coloured zones 

 also being found. Especially at the contact of a vein containing much 

 muscovite, there could be clearly observed how a long crystal 

 of tourmaline, which was interrupted in the marginal zone, continued 

 at some distance in the tine-granular quartz-tourmaline-mixture of 

 the adjoining rock, which points to a partly simultaneous crystalli- 

 sation of the vein and the adjoining rock. 



Like thob^e of the granitic area of Rokan the contact-phenomena 

 described above show a pneumatolytic character. The phenomena 

 in the first mentioned area point to such relations of pressure and 

 temperature and to such a percentage of mineralisers as make granite 

 apophyses possible to be formed in alternating layers with the 

 surrounding rocks, and these rocks to be imbibed with mineralisers. 

 The missing of felspathisation in the contact rocks of the hill 

 countries of Siak can be explained by crystallisation at lower tem- 

 perature and pressure, and a lower percentage of mineralisers (espe- 



