380 MR. J. B. SCRITEXOE OX THE [Dec. 1914, 



enough fluorine to convert all the nascent felspar into nascent topaz, 

 but it seems that there was enough lithia to change the nascent iron- 

 mica to nascent zinnwaldite. The chemical energy of the gases 

 was practically expended in the magma; and so, when the mass rose 

 to form the topaz-aplite, and, by segregation, the small quantity 

 of its modification rich in zinnwaldite, it had no entangled gases to 

 attack the porphyrinic granite or the quartz-topaz veins, except in 

 the possible cases of the schorl-rock bordering the pegmatite -vein 

 mentioned earlier as coming off from the aplite, the rich ore- 

 bodies in Quarry H, and the similar ore-body in the Chinchong 

 Valley. These ore-bodies must be admitted to constitute a diffi- 

 culty in the hypothesis. It is possible that the aplitic magma in 

 its upward course redissolved part of the quartz-topaz rock, or other 

 consolidated parts of the magma containing tin-ore, and that the 

 resulting gases formed the ore-bodies ; but the non-occurrence of 

 topaz in them is an objection to redissolved quartz-topaz rock 

 bein^' the source of the rich tin-ore. 



VII. Summary. 



The main points of this paper may be summarized as follows : — 



G-unong Bakau is a peak 4426 feet high in the Main Range of 

 the Malay Peninsula. It is composed of porphyritic granite, into 

 which have been intruded veins of quartz-topaz rock, and, at a 

 later date, masses and veins of topaz-aplite. 



The quartz-topaz rock has quartz and topaz as constant con- 

 stituents. Other important constituents, which, however, are not 

 always found, are cassiterite, zinnwaldite rich in iron and with the 

 axial figure of a uniaxial mineral, and tourmaline. The zinnwaldite 

 is only known to occur in considerable quantity in one vein. Else- 

 where it is sometimes found forming patches in the quartz-topaz 

 rock. 



The topaz-aplite contains a small amount of cassiterite. 



Where the quartz-topaz veins cut the granite, a 'reaction- 

 border ' of schorl-rock, and in one case of greisen, is found. These 

 reaction-borders differ widely from the veins themselves. 



Evidence is given in detail, showing that the quartz-topaz vein- 

 rock is not an alteration-product of a pre-existing rock, but was 

 intruded as a quartz-topaz magma. 



Ore-bodies formed by pneumatolytic alteration of granitic rocks 

 were once worked on Grunong Bakau, and they differed markedly 

 from the quartz-topaz rock. 



It is believed that the difference between the familiar pneumato- 

 lytic products, schorl-rock and greisen on the one hand, and the 

 quartz-topaz rock on the other, is that in the former case rocks 

 that had consolidated on the edge of a granite-mass were altered 

 by media coming from deeper parts of the mass; whereas in the 

 latter an accumulation of similar media attacked part of the still 

 molten magma deep down in the igneous mass, and the heat 

 generated by the reactions that took place caused the portion of 



