186 THE MINERALS OF TASMANIA. 



below. The Mt. Bischoff rock is essentially a vein-rock, and 

 we are disposed to refer it to the elvan group as a topaz- 



ised elvan-rock (now topaz-quartz-porphyry) 



In the Mt. BischofF rock the felspar of the groundmass has 

 been replaced by topaz. The analysis recorded by Von 

 Groddeck showed no alkali, and the rock consisted practic- 

 ally of quartz and topaz ; but this would naturally be the 

 case in parts of the rock when the topazisation process had 

 proceeded to its ultimate stage. In certain of our slides 

 the substance as well as the form of the felspar has sur- 

 vived, and we are thus able to diagnose the original rock 

 as containing porphyritic crystals of quartz, felspar, and 

 mica floating in a groundmass which is sometimes com- 

 posed of granular allotriomorphic quartz, sometimes of 

 crypto-crystalline or felsitic matter, but usually besprinkled 

 with scales of aluminous talc, derived from felspar and 

 mica. When the dykes contain less topaz, as on the North- 

 Valley side, we have detected a felsitic groundmass. We 

 may here mention that the survival of felspar is a rare 

 occurrence. What petrological observers have seen hitherto 

 have been crystal forms only ; and what is pointed out to 

 the visitors at the mount as kaolin is really a white decom- 

 posed product of pseudomorphous topaz and tourmaline. 

 The quartz phenocrysts are idiomorphic, sometimes with 

 perfect outlines, or with rounded corners and indentations. 

 Fluid inclusions are present, with fixed and moving 

 bubbles A very interesting feature is the con- 

 version of quartz to topaz, which is visible in hand speci- 

 mens. A quartz-sintery-looking rock, composed of quartz 

 in hexagonal prisms, shows its individual crystals bordered 

 with a white cloudy marginal zone of pseudomorphous 

 topaz. Heated in the open tube its vapour etched glass. 

 This topaz effervesces slightly when treated with H Cl, 

 owing to the unexpected presence of lime, derived possibly 

 from the alteration of sphene and apatite. We 

 witness here a second conversion — that of topaz into proso- 

 pite, a double fluoride of calcium and aluminium. When 

 this change is effected topaz loses its transparency, becomes 

 cloudy and opaque, its hardness diminishes, and its specific 

 gravity becomes less. Von Groddeck describes this jiseudo- 

 morphism fully in his paper, ' On the Tin-ore Deposits of 

 Mt. Bischoff, Tasmania,' 1886. Sandberger quotes this 

 rather peculiar mineral from Altenberg, Geyer, and Heng- 

 steverb, in S'axony^ and mentions that he has often 

 remarked pseudomorphoses of prosopite aggregates after 

 pycnite and crystalline topaz. Vauqulin had previously 



