Vol. 70.] TOPAZ-BEARIXG ROCKS OF GXXOXG BAKA.TT. 369 



of the rock. In other specimens, rich in cassiterite, it forms about 

 9 per cent, of the rock, with 9 per cent, of cassiterite. There do not 

 appear, however, to be any definite relations between the percentage 

 of topaz and that of cassiterite, as one vein was found rich in topaz 

 and dark mica, but with no cassiterite. 



The cassiterite is generally brown by transmitted lisrht. In 

 section it sometimes shows crystal boundaries, and in the crushed 

 rock crystals with sharp outline were found. In one section a 

 perfect little crystal was observed in quartz, and in another a perfect 

 crystal was seen apparently enclosed in topaz. The cassiterite does 

 not occur as veins or pipes in the quartz-topaz rock, but as a 

 disseminated mineral. In Quarry B it forms about 1*75 per cent, 

 of the rock. Occasionally one finds patches where the mineral has 

 segregated to form a higher percentage than this. It never forms 

 bisc crystals or bi°; grains. 



The tourmaline is not very common. Both brown and blue 

 tourmaline occur. In some veins the mineral can be distinguished 

 in hand-specimens as small dark spots. 



Patches rich in tourmaline were found in Quarry C, one of them 

 measuring 57 by 16 inches. In these patches the tourmaline 

 occurs as aciculate crystals, and is associated with quartz, small 

 clearly-defined ciystals of cassiterite, and topaz. Xo zircons were 

 found. 



The dark mica, which occurs throughout the exposed portion 

 of the vein in Quarry M, and as patches in other veins, is 

 greenish, brown, or sometimes almost black, in hand-specimens. 

 The axial figure does not open on rotation, so the mica is apparently 

 uniaxial. It is rich in iron, and every specimen examined spectro- 

 scopically gives a strong reaction for lithia. It may be called, 

 therefore, zinnwaldite rich in iron, and is comparable with 

 Scharizer's protolithionite. 1 Cassiterite commonly occurs with this 

 mica, and also topaz ; but it has been remarked that where the mica 

 is very abundant, the cassiterite is not abundant, and in some 

 cases is entirely wanting. Where the mica occurs as a patch, the 

 percentage of cassiterite in the rnica-patch is distinctly less than 

 in the surrounding quartz-topaz rock, where there is no dark mica. 

 The mica and the topaz are seen in section to be closely associated, 

 as though they had crystallized out from the molten vein-rock 

 together. 



The mica-patches vary considerably in size. One that I mea- 

 sured was 41 by 20 inches, but some are only an inch or two in 

 diameter. They contain quartz, topaz, and sometimes sulphides, 

 like the rest of the rock, and the only explanation that I can give 

 of them is that the mica was segregated in certain parts of the 

 still molten base. They differ from the familiar basic patches in 

 granite, in that the surrounding quartz-topaz rock contains no 

 dark lithia-mica ; whereas, in the granite, mica occurs both in the 

 patches and in the surrounding rock. 



1 J. D. Dana, ' A System of Mineralogy ' 6tli ed. (1892) p. 627 ; see also 

 ' Rabenglimmer,' ibid. p. 626. 



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