Vol. 70.] TOPAZ-BEAEIXG EOCKS OF GUXONG BAKAU. 371 



(4) Veins from the Reaction-Border invading the 

 Porphyritic Granite. 



In Quarry C a thin vein was found running off from the reaction- 

 border into the porphyritic granite. It consists of brown and blue 

 tourmaline,, quartz, zircon, and muscovite, and appears to be due 

 to the alteration of the granite along a fissure by media given off 

 from the quartz-topaz vein. A similar vein was seen on the ridge 

 south of the summit of Grunong Bakau, and another such vein 

 Avas noted in Quarry B. 



(5) The Border of the Quartz-Topaz Veins with 

 Dark Zinnwaldite. 



In Quarry M also there is a reaction -border between the vein- 

 rock and the granite. In part it resembles the reaction-border 

 described above, but generally the change effected in the granite is 

 of a different nature. Fortunately sections could be cut showing 

 both the altered porphyritic granite and the edge of the vein-rock. 

 The granite is mainly altered to a quartz-muscovite-rock, the 

 alteration extending only for a few inches at the most. Except 

 at the immediate junction the altered granite is very soft. The 

 muscovite occurs in felted masses of small flakes, and probably 

 represents the felspar of the granite. Owing to the softness of 

 the latter away from the immediate junction, and the general 

 kaolinization by weathering of the felspar, this could not be proved 

 by sections showing a gradual change. A little topaz was found 

 that may have been derived from the felspar also. In the altered 

 granite there is some altered biotite, as in the other type of reaction- 

 border, but only a little tourmaline. The edge of the quartz-topaz 

 vein is finer in grain than the body of the vein-rock. It contains 

 on the whole as much topaz as the body of the rock, and in some 

 cases more of that mineral. It o-enerallv contains more dark zinn- 

 waldite. In one specimen the zinnwaldite, which is brown and more 

 brightly coloured than in the body of the rock, but has been proved 

 to be the same species, forms a distinct band, evidently the result 

 of segregation during cooling. The greater abundance of the topaz 

 in certain specimens admits of the same explanation. Sometimes, 

 at the edge of the vein, one finds a beautiful intergrowth of the 

 zinnwaldite and granular topaz. The felted masses of muscovite 

 seen in the altered granite are not found in the border of the vein, 

 but occasionally there is a quantity of white micaceous material 

 formed by the alteration of the topaz. Tourmaline, generally of a 

 distinct brown hue. occurs at the edge of the vein-rock, though 

 not so abundantly as in the reaction-border. 



The difference between the junction of the vein-rock with the 

 granite in Quarry M, where the lithia-mica is abundant, and the 

 junction between the two rocks elsewhere, is that in the former case 

 the media coming off from the vein-rock produced a very thin band 

 of greisen, while in the latter the result Avas a more distinct band 

 of schorl-rock. 



