Vol. 70.] TOPAZ-BEABING BOCKS OF OUXOXG BAKATI. 367 



III. Description of the G-baxite a>~d Associated Rocks. 



(1) The Porphyritic Granite. 



The porphyritic granite is for the greater part weathered to a 

 soft mass, owing to the kaolinization of the felspar ; but in it are 

 numerous 'core-boulders,' some of great size, consisting of fresh 

 rock that has resisted weathering and affords good material for 

 study. Both the fresh and the weathered rocks were examined by 

 separation of the constituents in heavy liquids and by an electro- 

 magnet, while the fresh rock was also examined by means of 

 sections. 



The porplryritic crystals are of orthoclase. In the mass of the 

 rock are orthoclase and plagioclase, with maximum extinction- 

 angles that reach 23°. Enclosed in the felspar a little muscovite is 

 found. There is a little brown and also some blue tourmaline. 

 A very small quantity of cassiterite and topaz was found in some 

 specimens. In the fresh rock the chief mica is of a rich brown 

 hue, and is apparently uniaxial. Careful spectroscopic tests proved 

 a trace of lithia in this mica : the spectrum was very faint, but 

 distinguishable. This mica may be called biotite with a trace 

 of lithia. 1 Zircon and apatite are abundant. 



There is no definite evidence that the tourmaline, topaz, and 

 cassiterite, or any one of these minerals, are alteration -products of 

 previously-formed minerals ; but, in some specimens taken near the 

 junction with a quartz-topaz vein, the tourmaline and topaz are 

 more abundant than elsewhere. Both minerals in these cases may 

 have been produced by media coming from the quartz-topaz vein. 

 The muscovite enclosed in felspar may be an alteration-product of 

 that mineral, but I have no proof of this. It might equally well 

 be an original mineral, and the same must be admitted of the 

 cassiterite, topaz, and tourmaline. No topaz or cassiterite has been 

 seen in sections of the fresh rock, but topaz Avas detected by 

 crashing and separating a large quantity of the fresh rock. 



(2) The Quartz-Topaz Vein-Rock. 



Distribution. — Some idea of the distribution of this rock may 

 be gained from fig. 1 (p. 364). On the north one well-marked 

 vein occurs in Quarry M. At the summit of the mountain and near 

 the summit, as also along the ridge to the south as far as Quarry K, 

 there are numerous outcrops of veins. Exposures of veins are seen 

 in drives east of Quarry H, and one large circular section of a 

 vein is exposed high up on a face in this quarry. In Quarry T 

 another strong vein is exposed, and numerous smaller veins can 

 be seen in a cutting made for some mine buildings hard by (fig. 3, 

 p. 366). On the west the quartz-topaz rock is found as far down 



1 See J. D. Dana, 'A System of Mineralogy' 6th ed. (1892) p. 630— 

 Analyses 6, 7, 29, & 30 ; also p. 631— Analyses 35 & 36. 



Q. J. G. S. No. 279. 2 c 



