36 THE MINERALS OF TASMANIA. 



dykes of quartz-porphyry occur in the neighbourhood of 

 the main seat of mining operations, but that known as the 

 Queen dyke is by far the most important from a mining 

 standpoint. The topazisation of this has proceeded to the 

 fullest extent, and in places remarkable impregnations of 

 radiating acicular pycnite abound, while enormous masses 

 of a peculiar green tourmaline are strikingly plentiful, as 

 well as various pyrites and (in a minor degree) fiuorite, 

 siderite and prosopite, while wolframite is an extreme 

 rarity. From a mineralogical standpoint this remarkable 

 tin mine shows a relationship to the Schneckenstein of 

 Saxony, inasmuch as several of the usually rarer minerals 

 are common to both localities. At both places there 

 are the advanced topazisation of the porphyry, the abun- 

 dance of the cylindrical form of that mineral known as 

 pycnite, and the occurrence of prosopite, the last being 

 almost peculiar to the two localities mentioned. The cas- 

 siterite of Mt. Bischoff is not noted for its fine develop- 

 ment, as it mainly occurs in remarkably small crystals, 

 and much of it is in the form of actual slime in friable 

 topaz. Cassiterite crystallises in the tetragonal system, 

 and forms one of a group which includes the dioxides of 

 tin, manganese, titanium and lead. They all belong to 

 the same system of crystallisation, with closely similar 

 angles and axial ratio. Cassiterite crystals are very com- 

 monly mackled, and not rarely show geniculate or 

 knee-shaped forms. Zonal structure is often seen in 

 microscopic sections of the crystals. This is well illus- 

 trated by the beautiful slides produced by sections of the 

 pycnite of Mt. Bischoff, containing, as it usually does, 

 associated tin crystals. The late Professor Ulrich states 

 in Jifej'is: — "The crystals of this mineral (cassiterite) 

 occurring at the Mt. Bischoff Tin Mine are in the average 

 small, rarely exceeding 5 mm. in size. They are gener- 

 ally also not well developed, having rounded edges and 

 often broken faces. Their form is very simple ; generally 

 the unit pyramid and unit prism, with rounded-off edges, 

 indicating an octagonal prism. Twinning according to 

 the common law-twinning and composition plane, the 

 diametral pyramid is very frequent."' The East Coast 

 tin-mining area affords masses in places of more or less 

 altered granite, containing an appreciable quantity of 

 cassiterite. Much of this rock is converted into greisen — 

 a quartz mica rock commonly associated with topaz and 

 tourmaline. The ore-bearing veins appear in most cases 

 to be altered rock, impregnated with cassiterite and the 



