Vol. 70.] TOPAZ-BEARING ROCKS OF GT7XOXG BAKATJ. 373 



The other case is in the underground workings of Quarry M. 

 Here the junction of the topaz-aplite with the porphyritic granite 

 is marked hy a border, about half an inch thick, of pegmatite. 



(7) Ore-Bodies believed to have been formed by Media 

 from the Topaz-Aplite Intrusions. 



Except in one spot, the topaz-aplite has not proved to be worth 

 working for tin -ore, but in Quarry H certain ore-bodies were found 

 some time ago, and one was found in the Chinchong Valley (see 

 above, p. 365). Except for specimens, these ore-bodies have all dis- 

 appeared now ; but it was evident that they were the result of the 

 alteration of pre-existing granitic rocks, and probable that the 

 media which effected the alteration came from the aplite-intrusions. 

 The evidence on the latter point is not so clear as it might be, but 

 it need not be discussed here, as the ore-bodies are merely men- 

 tioned in order that emphasis may be placed on certain points 

 that mark them off as distinct from the quartz- topaz rock. 



In the first place no topaz was found in them. They contained 

 little quartz, but a large amount of pale-green mica, which is 

 practically colourless in thin sections. Where the ore was very rich 

 the rock consisted of nothing but cassiterite and mica. Where it 

 was poor, in Quarry H, it was felspathic, and two micas Avere 

 distinguished : a mica with calcite as an alteration-product and the 

 figure of a uniaxial mineral, believed to be the original mica of the 

 rock in which the ore-body was formed ; and a biaxial, secondary 

 mica, largely developed in the felspar of the original rock. Xone 

 ■of the mica from these ore-bodies in Quarry H gave a reaction for 

 lithia. In the ore-body found in the Chinchong Valley, the 

 powdered rock gave a very faint reaction for lithia, which was 

 believed to come from a small quantity of mica having the axial 

 angle of ordinary zinnwaldite. The rest of the mica in this ore- 

 body had a small axial angle, excepting a trace with the figure of a 

 uniaxial mineral, that might have been the original mica of the 

 rock in which the ore-body was formed. 



These ore-bodies then, formed by the alteration of 

 granitic rocks, had no topaz, and with the exception just 

 noted, no lithia-mica: but they contained abundant 

 secondary mica that Avas distinctly biaxial. 



The quartz-topaz veins and the topaz-aplite, on the 

 other hand, have abundant topaz, no secondary mica 

 (except that formed by the weathering of the topaz), 

 Avhile they do contain a dark lithia-mica, rich in iron, 

 that has the habit of an original rock-constituent. 



IV. The Faults. 



There are numerous small faults cutting the granite, quartz- 

 topaz veins, and aplite of Gunong Bakau. The amount of throw 

 is small. The largest throAV that I could measure Avas less than 



