Vol. 70.] TOPAZ-BEABIXG EOCKS OF GUXOXG BAKAU. 375 



fissures have been observed, and the vein-rock, rich in topaz, ends 

 abruptly against the granite, which, however, has been altered for 

 a few inches by emanations from the vein-rock to form familiar 

 pneumatolytic modifications. 



(2) If the quartz-topaz rock is an alteration -product, then we 

 must assume that the reaction-borders of schorl-rock and greisen 

 mark the farthest extent to which the alteration reached. Then 

 why is the vein -rock so dissimilar from the rock that is clearly an 

 alteration-product ? It should be the same. Where the reaction - 

 border of schorl-rock occurs, we should have a solid vein of nothing* 

 but schorl-rock. Where the reaction -border is of greisen, we 

 should have a vein of nothing but the same rock. To put these 

 objections in another form : — 



(a) Why does the proportion of topaz, so abundant in the vein, suddenly 



drop almost to vanishing-point in the reaction-border ? 



(b) Why does the cassiterite, common in the vein, practically disappear 



in the reaction-border ? 



(c) Why are not the masses of white mica that are found in the reaction- 



border found also in the vein-rock ? 



(d) Why is tourmaline so abundant in the schorl-rock reaction-border, and 



not abundant in the vein, from which the schorl-rock is separated by 

 a sharp line of division ? 



(e) Why are zircons common in the schorl-rock reaction-border and so rare 



in the adjacent vein-rock ? 



(3) If the quartz-topaz rock is an alteration -product, then the 

 iron-rich zinnwaldite is an alteration-product, as it is not the same 

 mica as that in the granite. Then it must have been formed from 

 something. In greisen the secondary mica is believed to have 

 been formed from felspar. But in Quarry M we observe masses of 

 white mica in the altered granite, formed from felspar in all pro- 

 bability, and the zinnwaldite is distinct from this mica. Moreover, 

 the zinnwaldite has included zircons with pleochroic halos. Were 

 these zircons originally in felspar-crystals waiting for the forma- 

 tion of the zinnwaldite ? 1 



(4) If the zinnwaldite originated by alteration of some mineral 

 in the granite, since the granite is much the same in composition 

 throughout the area under discussion, why do we not find the 

 zinnwaldite in all the veins ? and why do we find it in isolated 

 patches ? 



(5) Why in Quarry M do Ave get finer grain at the edge of 

 the vein, suggesting quicker cooling, and sometimes a higher per- 

 centage of zinnwaldite and topaz suggesting a segregation of these 

 minerals in the part of the vein that first became solid ? 



(6) Finally, why were the ore-bodies mentioned above as oc- 

 curring in Quarry H and in the Chinchong Valley, ore-bodies that 



1 In E. Beck's 'The Nature of Ore-Deposits' (Engl, transl. by W. H. 

 Weed, vol. i, p. 200) a greisen from Banka is figured showing ' mica with 

 dark aureoles round zircons.' This figure resembles closely sections of stone 

 from the vein in Quarry M on Gunong Bakau. It is evidently held to be an 

 alteration-product {op. cit. p. 201), but how the zircons got into the lithia- 

 mica is not explained. 



