368 mr. j. b. scritexor ox the [Dec. 19 14,. 



from the summit as 460 feet and at a distance of about a mile. On 

 the east, in Quarry B, is a strong vein about 15 feet thick, cropping 

 out 280 feet down from the summit. This has been followed into 

 the hill by means of drives. It appears to extend southwards 

 towards Quarry C, where also is a thick vein, which may be the 

 same as that in Quarry B, or a parallel vein at a higher level. Near- 

 Quarry C is evidence which leads one to believe that there may 

 be three other veins, one cropping out below the other. Far to 

 the south, about 2 miles away, the quartz-topaz rock has been 

 found again, on the slopes of G-unong Raja, both by myself and 

 by Mr. W. E. Jones, who also discovered a similar topaz-bearing 

 rock near the foot of the Main Range in Selangor, some 10 miles 

 distant. 



Owing to the fact that the quartz-topaz rock does not weather 

 readily, because it contains no felspar, the veins are easily distin- 

 guished from the soft, weathered porphvritic granite. (PI. LI, 



fig.i.) 



Petrology. — When quite fresh the pure quartz-topaz rock is 

 pale in colour, and of medium grain. It is hard. When exposed 

 at the surface, it becomes slightly porous and pure white. This is 

 owing to the slow solution of the quartz-grains and the alteration 

 of the topaz to a fine, white, micaceous substance. No evidence 

 has been found of the topaz weathering to kaolin. Masses of 

 practically pure quartz-topaz rock are exposed on the south side 

 of G-unong Bakau. 



Quartz and topaz are, as already stated, constant constituents. 

 Other constituents are cassiterite, tourmaline, arsenopyrite, pyrite, 

 and white mica. The last is rare, and not noticeable in a hand- 

 specimen, some being a product of weathering of the topaz, but 

 some certainly an original mineral of the rock. Its scarcity may 

 be iudffed from the fact that, although I watched the crushed stone 

 from Quarry B passing over a Wilfley table for some time, I could 

 not detect any. Sections of the rock show an occasional small flake. 

 In Quarry M, however, there is throughout the rock a dark mica, 

 easily distinguishable in a hand- specimen. The same mica occurs 

 in patches elsewhere in the quartz-topaz rock. A little fluorite has 

 been found in a section, and, as a result of treating large quantities 

 of crushed rock, two zircon-crystals were detected. The signifi- 

 cance of the extreme rarity of zircon will be dealt with later. 



The quartz is in irregular grains, and calls for no special 

 comment. 



The topaz cannot be distinguished easily in any hand-specimen 

 that I have seen. In sections under the microscope it is seen to 

 have some straight boundaries, and in separations of crushed rock 

 it sometimes shows fair crystal outline. It frequently occurs as 

 minute grains enclosed in quartz. The cleavage parallel to (001) 

 is wrll shown in sections. In some specimens, where there is little 

 or no cassiterite, the topaz was found to constitute about 5 per cent. 



