184 THE MINERALS OF TASMANIA. 



evidently been formed in veins of pegmatite, which traverse 

 the granite, and vary from 1 to several feet in diameter 

 The colour varies from pure limpid to various shades of 

 blue, pale-pink, yellow, &c. Crystals are found up to sev- 

 eral inches in diameter. A fine crystal measuring 7 mm. 

 X 9 mm. X 7 mm., and perfectly clear and colourless, 

 was measured on the goniometer. As the faces are very 

 irregularly developed and one side of the crystal is broken, 

 the crystal is drawn in ideal symmetry, but so as to show 

 the habit as nearly as possible. The prisms m (110) and 

 V (120) are about equal in size and striated, but the images 

 are good. The brachydomes / (021) and" y (041) are 

 relatively small, while the macrodome d (201) is unusually 

 large and brilliant. The base is large and smooth. The 

 pyramid o (21) is small, // (HI) and i (223) large and 

 brilliant." 



From Bell Mount, " Topaz occurs at Bell Mount in a 

 very decomposed quartz-porphyry, also as pebbles weath- 

 ered out in the drift ; it has not previously been recorded 

 from this locality. Two crystals, both colourless and 

 transparent, were examined ; one is much worn 

 and broken, and unsuitable for goniometric determination. 

 The other has good prism and dome faces, but the pyramids 

 are dull, and were measured in the position of maximum 

 illustration. The base is absent. The crystal measures 

 13 mm. X 10 mm. x 11 mm. The coordinate angles are 

 calculated in all cases." 



Variety — Pi/cni f e . 



This mineral is distinguished from normal topaz by its 

 columnar habit and very compact structure. The fracture 

 is conchoidal to uneven ; it is translucent at the edges, with 

 a vitreous lustre on the cleavages. It is pyro-electric, 

 infusible b3fore the blowpipe, but blist-ers more readily 

 than topaz proper. In hardness it is 7*5, that of true 

 topaz being 8. It has the faces a m, and a perfect cleavage 

 parallel to the face c, with planes of union sometimes 

 parallel and sometimes making an angle with c. It is 

 found in parallel columnar aggregates at the tin mines of 

 Altenberg in Saxony, and of Schlackenwald, Bohemia. 

 The occurrence of the local variety of topaz was apparently 

 easily recognised as occurring at Mt. Bischoff, but was not 

 recorded. The topaz-quartz-porphyry of this locality is in 

 all probability the most extensive, and mineralogically the 

 finest development, of the variety pycnite of topaz proper 

 that has hitherto been investigated. 



